We have the same issue. Its intermittent. We had complete failure a month or so after porting where outgoing calls were on the old number and incoming weren't received. That was sorted but since we've had intermittent times when the phone doesn't receieve calls or texts. Worked fine on bt mobile (also ee) before with no issues. It appears this is not an isolated issue as there are other threads like this on ithttps://community.plus.net/t5/Mobile/Not-receiving-incoming-calls/td-p/1407947/page/3
I think we'll just move on to another network as I don't have time to spend chasing to sort out phones again.