I'm a newbe here,
Well I dunno why kelerman's post hasn't been removed. I think I may not be on a very good forum.
However......I've spent hours trawling the net to overcome this problem. I tried the sim in another phone - same problem which makes one suspect the network. Phoned EE support - a really cheery and willing guy called Eric from Waterford. With what I had gained from my research and his encouragement I found the answer. The SMS messaging centre's number was set to my own phone number so instead of being sent on I was unsuccessfully trying to send messages back to myself. On my OnePlus 3 the SMS information is very well hidden and don't think you'll find it in any settings on the phone. What you have to do is to "phone" *#*#4636#*#* You will get a page labelled Testing with three sub headings the top one is Phone information. Click on this and you get a lot of information; scroll down until you get to SMSC:. In here you have to put inverted commas followed by the telephone number of your SIM supplier (Get it from them - unless you can find it on the Internet) followed again by inverted commas. Click the update button and, in my case, it added a comma and 145 to the SMS number still in inverted commas. I'm not sure if having my own phone number in Messages>three vertical dots at top right>Settings>Advanced>Phone Number caused this and if you manage to fix the problem by putting the correct SMS messaging centre number in there I'd be pleased to know.
Glad to have solved the problem and hope this helps others.
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