It will be because you are in a weak 3G area. When on wifi your phone will connect to a 2G network (as it doesn't need 3G for data) but will try to find a 3G signal when wifi is off. 2G signal (calls & texts) is always better than 3G.
I have no affiliation whatsoever with O2 or any subsidiary companies. Comments posted are entirely of my own opinion. This is not Customer Service so we are unable to help with account specific issues.
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