I have Iphone 7 and I has happened since day one (usually I send Imessages and it's fine). I have tried:
1- reset network;
2 turn phone off/ on
3- called *#5005*7672# (I found it at Fido cimmunity) and I got the message "error performing request Unkow error".
Welcome to the community!
Firstly, I am personally not that familiar with iPhones. However, this is some information I've come across in my research on your situation. I'm not sure if that code to display the short message service centre (SMSC) works in later iOS versions (see here). Have you tried to change the SMSC? As far as I can tell, the code to change the number still works. Judging by the informationtion posted here, to change the number you can use: