SMS delivery bug after transferring phone number between carriers

[Axk]

Is there anyone here who has insight into how SMS messages are delivered internationally?
The infrastructure, gateways, routing etc.
There is this commonly known issue that when one transfers their phone number to a different mobile provider texts from Google and other online services like Office 365 stop arriving.
Is this some known bug in some software along the SMS delivery route?
Is this an issue on Google's (Microsoft's) side or the carrier's side?

[Halcyon]

I can't say I've ever come across or heard about this issue. I've had no issues with these kinds of messages on numbers ported from one carrier to another.

[NiHaoMike]

Could the sending server be using a cached destination IP or hostname that's no longer current?

[tooki]

The issue could be that many online services check numbers against some kind of database that tells them whether it’s with a “real” telco as opposed to on a virtual service (like Google Voice) to prevent registration with disposable numbers. I transferred my US number from AT&T to google voice and I can no longer use it to verify a handful of things.

[tooki]

Could the sending server be using a cached destination IP or hostname that's no longer current?

SMS is not IP based.

[LapTop006]

Could the sending server be using a cached destination IP or hostname that's no longer current?

SMS is not IP based.

Inter-carrier SMS usually is. Also the older standards are pre-Unicode (well, pre universal usage of Unicode), and can have trouble not mangling even pretty basic text. I'd hope such things are long-obsolste, but they have a way of sticking around.

[tooki]

Could the sending server be using a cached destination IP or hostname that's no longer current?

SMS is not IP based.

Inter-carrier SMS usually is. Also the older standards are pre-Unicode (well, pre universal usage of Unicode), and can have trouble not mangling even pretty basic text. I'd hope such things are long-obsolste, but they have a way of sticking around.

Yeah, but that’s irrelevant to the situation, since IP addresses/hostnames aren’t per-number.

[IDEngineer]

May not be relevant, but: We are in the process of "freshening" our family's cellphones and one we are considering is the OnePlus 6T. It has an LTE transceiver but no CDMA transceiver, and it is certified (but not sold) by Verizon for use on their network. When we fired it up, SMS worked outbound but not inbound. Turns out that Verizon still defaults to CDMA for at least some portions of SMS messaging. We called Verizon and had them enable a function called (IIRC) "CDMA-less roaming". Once that was enabled SMS instantly started working on the 6T, because SMS was then forced to use LTE rather than the CDMA default. (BTW, apparently Verizon is the last remaining carrier to use CDMA so more phones may be discarding support for it and we may be seeing this problem crop up more frequently.)

So... you might investigate if the hardware in question has a different radio setup and talk to your carrier about the specific phone model in question. Just an idea.

[grizewald]

My guess is that there is something wrong with the SMS setting called "Message Centre".

If I'm not mistaken, the message centre number is sent as part of the SMS you send so that the receiving phone knows how to reply to the message. (It's probably also included in a delivery receipt.) The message centre number will be different between operators, so either Google and Microsoft are caching the old message centre number, or your phone still has the old carrier's number.

[Halcyon]

My guess is that there is something wrong with the SMS setting called "Message Centre".

If I'm not mistaken, the message centre number is sent as part of the SMS you send so that the receiving phone knows how to reply to the message. (It's probably also included in a delivery receipt.) The message centre number will be different between operators, so either Google and Microsoft are caching the old message centre number, or your phone still has the old carrier's number.

If outbound SMS's were not being delivered, I'd say that the SMSC is incorrect. But if they are being sent but not received, the problem is elsewhere. Normally, the SMSC number is stored on the SIM card so manual changes to the phone settings shouldn't be required.

[LapTop006]

(BTW, apparently Verizon is the last remaining carrier to use CDMA so more phones may be discarding support for it and we may be seeing this problem crop up more frequently.)

Not quite, Sprint in the US as well. Most of the rest have been shut down by now.

[Tahifter]

I've done a bit of digging, and it seems that the problem might be on the carrier's side, especially after number porting. When you transfer your number, the routing information needs to be updated across various systems, and sometimes there can be a lag or error in this update process, causing SMS messages from certain services to get lost or delayed.