Iphone not sending SMS
I have recently purchased an iPhone 6s plus unlocked for gsm carriers. I am only having problems with my girlfriends SIM card which it will not send text messages, it will do everything else, receive sms, make and receive calls it will even send mms. When I put my SIM card from my phone into it, it will send sms just fine. When I called my carrier they said it was an apple issue, but I'm not totally convinced. Has any one encountered this issue before?
iPhone 6s Plus, iOS 11.2.6