In iOS 15, Apple added support for iPhones to support SMS over IMS networks – SMSoIP. Previously iPhone users have been relying on CSFB / SMSoNAS (Using the SGs interface) to send SMS on 4G networks.

Getting this working recently led me to some issues that took me longer than I’d like to admit to work out the root cause of…

I was finding that when sending a Mobile Termianted SMS to an iPhone as a SIP MESSAGE, the iPhone would send back the 200 OK to confirm delivery, but it never showed up on the screen to the user.

The GSM A-I/F headers in an SMS PDU are used primarily for indicating the sender of an SMS (Some carriers are configured to get this from the SIP From header, but the SMS PDU is most common).

The RP-Destination Address is used to indicate the destination for the SMS, and on all the models of handset I’ve been testing with, this is set to the MSISDN of the Subscriber.

But some devices are really finicky about it’s contents. Case in point, Apple iPhones.

If you send a Mobile Terminated SMS to an iPhone, like the one below, the iPhone will accept and send back a 200 OK to this request.

The problem is it will never be displayed to the user… The message is marked as delivered, the phone has accepted it it just hasn’t shown it…

The fix is simple enough, if you set the RP-Destination Address header to 0, the message will be displayed to the user, but still took me a shamefully long time to work out the problem.