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There have been previous threads on the forum asking this question, it may be worth having a look around the search facility, you should be able to find some previous helpful replies.
The first question to ask is the make of phone you are using? It is difficult to give an answer without knowing if you are using Apple or Android.
This is probably due to the phone still using details of the temporary number used prior to porting. Try sending the text as new and not part of the original conversation.
If you are using iphone, delete and previous stored memory from your icloud account and phone and as on the first linked thread, make sure the phone is showing the correct number.
However, you will probably be looking at a phone factory reset for Android or a restore for Apple this will clear the phone of all previous stored memory for the previous number used.
Thanks, but all sorted now, i took the sim out a few times and then dropped it on the floor, reset the phone four times....swore a bit and now it works.