You’ve transferred your mobile phone number from one mobile network operator (also named provider or carrier) to another. The new one gave you a SIM card from the first day of the new subscription, but until the phone number transfer was actually performed you’ve got a temporary number associated with it.
In the meantime you have also configured and started using an iPhone. Until the transfer occurred you used (or didn’t use) the temporary phone number for calls and SMS-es and it worked. After the transfer you could use your original phone number both for calls and SMS-es instead. Maybe with a short supplemental delay in some cases, such as for activation codes from online service providers (12 hours or around), due to caching.
But even if you have waited for an entire week, powering off and on the phone and/or removing and inserting the SIM again to test, you have seen that you couldn’t change the phone number displayed in iMessage and FaceTime apps. They both access the number from Phone settings, where the original temporary number of the second provider is still displayed. You have probably saw that you can change My number from settings trying to set the correct one, but if you turned off iMessage and FaceTime and back on, or if you simply navigated to My addresses in their settings or just waited some time, the unwanted temporary number always came back, again and again.
Contacting the support department of the second provider would be your next step, trying to resolve the issue, right? OK, but do NOT reset the phone to factory settings, if they say this is the only solution, like they tried to fool me for an unknown reason – they were just stupid of maybe they envied me that I had an iPhone 7? 🙂 – I haven’t accepted!
Instead, you should request from them to change your SIM card. Since now your original phone number is fully on their network, it might resolve the problem. Yes, you do have that right – you may lose or deteriorate the SIM so they need to do it in such cases. But do ensure that before getting the new one you disable iMessage and FaceTime – just in case.
This worked for me perfectly: immediately after switching the SIM I saw 0 as my phone number and could edit the value, and eventually, when re-enabling them, iMessage and FaceTime apps detected it and have bound themselves to the correct number too! Yay! Now I just need to find friends to properly use them with. 😀
This demonstrated to me that either the temporary phone number was stored on the SIM card itself (like I read on some forums) or that iOS never retried reading the phone number from the card again if it was the same card (maybe it has an internal dictionary of phone numbers per SIM card IDs; I haven’t checked with a different SIM to see). My personal feeling is, however, that the first case applies here – and that the support guy was simply uninformed. (I will write a review for that soon…)