They can be several reasons: hardware problems phone ( . It goes wrong , it's wrong porthole sim, etc. ) , problems Phone Software ( misconfigured APNs , who is not doing well the firmware , the modem is not the right , etc. . ) , attendant problems ( signal , net, antennas , maintenance, etc. . ) .
Therefore, in order to guide the issue is important that before you do a couple of tests to be discarded if it is antennas , software or hardware to test the chip in another phone, try another chip into your phone, put on and take airplane mode , try to remember when and where there are problems, what was done before you start to notice problems .
When you've done that , only then likely to make wipes ( see FAQ Modding below), change MODEM ( see FAQ Modding below), change firmware ( see FAQ Modding below), change phones by going to guarantee or change phone company .
In addition , you have to tell what firmware you have (if you touched it or not , if stock , attendant , etc. . ) .
If you are using the search engine (and here we're going to be repetitive ) you will find many threads with this kind of problems and solutions .