Carrier I can send but not recieve texts

Yesterday I completed an upgrade on an iPhone 4 to an iPhone 5c whilst switching the carrier from EE to o2. It has now been over 24 hours and I have been able to send texts to anyone and they've received them but I am unable to receive any potential replies via non-iMessages. Why is this happening and what should be done to stop it?

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I must add that I have transferred numbers across from the 4 to the 5c...