I have had this on Giffgaff and Voxi, the only solution I have is deselect your number if it is already trying to activate and obviously not getting anywhere and wait about a week then try it again, iMessage seems to clearly struggle when numbers have been ported and not with their original network. I had tried everything and Apple blame the network and the network blame Apple. It seems more common to be problematic when a SIM has been known to the phone at some point to have a different number before the porting. Hopefully it is a bug they resolve eventually, iOS 18 is due out very soon and there might be something in there. I would be interested to see what the iPhones send in that SMS to activate iMessage, I presume it is devices reported MSISDN (has to match the number the text is received from on their server), IMEI and maybe some other identifiers. I do not consider it to be a network issue at all as all they can check is SMS is being sent and received (if that works it is not a network issue). The iMessage and FaceTime activation number is a UK mobile number (there are multiple) so activating it abroad could be an issue with roaming charges etc. I feel it is a software glitch somewhere getting confused between temporary and ported in number, my advice with port ins and iOS is keep your existing SIM in the phone until is stops working and use a non iOS device on that day until you are fully aware everything is functional. I find on my device siri keeps finding my temp number in settings despite it not being present anywhere so I feel that is part of the glitch here. I seriously hope Apple find a solution as I feel they are to blame but trying to get an answer out of them is impossible. The support teams only know as much as you can find out online and read off a script.