How Number Forwards Work
Overview & Terminology
When setting up a call forward, you will need to specify two numbers: the forwarding number, and the forward destination. The carrier currently in possession of the forwarding number (the forwarding carrier) needs to set up the forward. Callers will dial the forwarding number (leg A), and the phone switch responsible for this number will originate a new call to the forward destination (leg B). Once both legs are established, the phone switch will bridge these two calls. Technologically speaking, neither party will know for sure that the call has been forwarded: only the switch, and by extension the forwarding carrier, will be able to see this explicitly. The forwarding switch will proxy all audio and handle the signaling on each leg independently.
The calling party can also still dial the forward destination directly. This allows the call to traverse the optimal path along the telephone network. When using a number forward, the call legs may be independently optimized to a certain extent, but only insofar as all calls must pass through the forwarding switch. This often leads to suboptimal call routing, significantly increasing the complexity of the call path and often reducing call quality.
Forwarding vs. Porting
Porting a number is the preferred long term alternative to number forwarding. This permanently moves a number from one carrier to another, so all calls to that number are direct. This reduces the complexity of the call path and generally means you will end up paying significantly less per minute as fewer charges are incurred. You can learn more about the porting process here. The port-in process may take a while, so in the meantime you can have the losing carrier forward the number you would like to port to a temporary T38Fax number and request a caller ID change on your trunk from T38Fax support.
Setting up Forwarding on T38Fax
Previously customers were required to call into support to configure a number forward, but our self-care portal has been significantly enhanced to support multiple users and self-serve number forwarding. T38Fax offers two types of forwards, which work slightly differently.
Cost of Forwarding
Per-Call-Minute Fees
Our standard per-call-minute fees apply to call forwards, though at minimum a forward will generate two calls, representing the two forward legs. You should also anticipate paying a per-call-minute fee to the forward destination's carrier.
For instance, if you're forwarding one T38Fax number to another, you will have to pay a per-call-minute fee for:
- the call inbound to T38Fax's forwarding switch,
- the call outbound from T38Fax's forwarding switch to the forward destination, and then
- finally, the call coming into the forward destination's trunk.
This is 3x the cost of reconfiguring your setup to accept calls to the forwarded number directly, so we recommend reconfiguration over a forward where possible.
DID Fee
The standard monthly DID ownership fees also apply to forwarded numbers.
Types of Forwards
Number Forward
Number forwards are are generally what you will want to use for most forwards. When you set up a number forward, you can pick one or more non-trunk-ID number you own to forward to any other number, on or off T38Fax's network. There is no monthly "trunk" fee for this service, which differentiates it from the trunk forward. When DIDs are released from their role as number forwards, the DID(s) will return to the trunk they came from, where possible.
Trunk Forward
A trunk forward allows you to forward an entire registration auth trunk to a single destination number. This type of forward is intended primarily for customers who provision a separate SIP trunk for each ATA-connected fax machine and want a simple way to deal with port-outs. The main benefit of this forward method is that your ATA's configuration does not need to change, and its registration will not be interrupted, in case you need to undo the forward later. The main downside of a trunk forward is that you will still pay the same flat monthly fee for this trunk as you would any other — so it will cost more in the long run than a number forward.
Testing your Forward
Before signing off on your forward, please remember to place a test call, preferably a test fax, to ensure your forward is working correctly. Double check that your forward destination is the correct one and that you can connect to the forward destination by calling it directly. Please also consider the possibility that there may be a forwarding loop. We do have some measures to caution or completely stop you from creating forwarding loops, but we can't be aware of all of these as it's possible to, for instance, create forwarding loops across carriers.
Forwarding Your Trunk ID
A DID serving as a trunk ID can not be used for number forwards. Forwarding your trunk ID therefore represents a special case of forwarding, which differs based on the type of your trunk.
For Reg Auth Trunks
Reg Auth trunks have effectively three options to forward the trunk ID:
- Use a trunk forward.
- Delete the trunk.
- Replace the trunk ID with another DID, possibly an arbitrary placeholder number.
A DID serving as a trunk ID can not be used for number forwards. Trunk forwarding provides a relatively straightforward and reversible solution for most trunk ID forwarding needs.
If you're porting out, you can also permanently delete your trunk and set up a number forward. Usually the cost savings on the monthly SIP trunk fee aren't worth it, though: deleting the trunk will erase your credentials and, even if you remember them, your SIP device may not automatically reconnect. This makes it far more difficult to reverse course if your project calls for it.
There may be situations in which you want to forward only the trunk ID and none of the others. In this case you will have to to change the trunk ID to something else. This can cause downtime, so proceed with caution. Since a reg auth trunk ID is the trunk's SIP username, changing it will require you to update the SIP device connecting to it as well. Until you update both your T38Fax trunk and your SIP device, you will lose service. For high-volume trunks using reg auth, it makes sense to allocate a dedicated placeholder number, owned by your organization and not any of your customers, for use only as your trunk ID. This not only reduces the chance of complications trying to forward this number, but also prevents a customer from porting out this number and interrupting the rest of your service.
For IP Auth Trunks
For IP auth trunks, the last DID on your trunk effectively serves as its trunk ID, and this DID will be referred to as the trunk ID throughout this document. IP Auth trunks have two options to forward the trunk ID:
- Delete the trunk.
- Replace the trunk ID with another DID, possibly an arbitrary placeholder number.
You may want to delete the trunk entirely if you do not anticipate the need to fall back. However, deleting the trunk will permanently delete all of your trunk settings, settings which can get quite complex for IP auth trunks. If you want to preserve your option to reverse your changes, we recommend replacing the trunk ID with another DID, possibly a temp or placeholder DID. The impact of changing the trunk ID for an IP auth trunk is minimal. You will have to pay the monthly DID fee for this number and continue to pay the monthly trunk fee, but it may be worth the cost depending on your circumstance.
Why Doesn't T38Fax Recommend Long-Term Forwarding?
As stated elsewhere in this article, T38Fax does not recommend call forwarding as a long-term solution. We recommend it mainly as a stop-gap when porting a number, so in almost all cases less than a month. Call forwarding is a technology that works great until it doesn't — and then your options for recourse are unusually challenging. This section covers these challenges in a bit more detail. If it sounds complicated, it's because it is!
Disadvantages of Forwarding
T38Fax does not recommend number forwards as a long-term solution, both from a financial and technical perspective.
From a financial perspective, there will be additional fees with number forwarding versus direct calling. You will have to pay for a second number. It's also typical for call forwards to cost the standard per-minute usage charge for both forwarding legs individually, plus any per-minute fees associated with the forward destination separately. This results in you as the recipient having to pay for 3x the number of minutes compared to a direct call!
From a technical perspective, number forwards force sub-optimal call routing and significantly increase the complexity of the call path. This generally increases your expenses and also leads to call quality degradation. In other words, expect to see a higher fax failure rate when using a number forward. These are technical limitations present across the telephone network and are not specific to any one carrier. Many voice carriers also discourage call forwarding, but it is especially important for fax.
Consider a couple of ways in which this suboptimal routing can manifest due to a call forward:
- Suppose there is an issue between the forwarding switch and the forward destination (leg B). If calls were direct, this issue may have been too negligible to notice, perhaps only occurring in the vanishingly few calls that happen to need to pass through the forwarding switch organically. However, since all calls must pass through the forward, this otherwise negligible issue turns into a full outage for that number.
- Separately, suppose you are forwarding calls from one carrier to another. This time, there is an issue between the caller and the forwarding carrier (leg A). You decide to only open up a ticket with the forward destination's carrier. Unfortunately, there's nothing the forward destination's carrier can do to resolve the issue. In fact, they likely don't even know that the call path includes a forward. Any suggestion they can provide from this point on will almost certainly not work, and will not necessarily be any faster to try than porting the forwarded number into their service.
Troubleshooting Forwards
When it comes time to open a ticket, please open a ticket with both the forwarding carrier and forwarded-to carrier. Fully explain the nature of the forward to both carriers.
Forwards will add additional parties and technical complications to the call path. When troubleshooting a call forward, the first step should be to try to isolate the issue as much as possible. Often this can lead to several parties being removed from the conversation entirely, which makes further troubleshooting a lot simpler.
You can typically determine which leg is at fault by determining whether all callers or just a small handful have a problem. While not always a 100% accurate metric, it is a good place to start. Generally you can determine this information from fax logs. T38Fax can also send calls over multiple carriers to determine the scope of the issue.
See the diagram below for a visual. If all callers have a high failure rate, there is likely an issue somewhere between the forwarding switch and the forward destination. This is because the routing between the forwarding switch and the forward destination is going to be fairly homogenous across all numbers. By contrast, the routing from the caller to the forwarding switch will still be quite diverse, particularly if the callers are on different carriers themselves. Thus, if only one or a few callers are failing, it's probably something between the caller and the forwarding switch.
These are just best guesses, and can be refined through further testing.
We also recommend having the caller place a direct call to the forward destination. This can help rule out whether the sub-leg between the forward destination's carrier and the fax endpoint are at fault. Even when only some callers are failing, having the caller try the forward destination directly can help determine whether this is an endpoint issue, for instance whether the fax machines are able to agree on fax parameters and so on. This test can also reveal additional options to solve your problem that don't involve troubleshooting the forward: perhaps it's simpler to port the number?
