Try to update the firmware first then if it does not solve the issue try performing a factory reset but don't forget to back up all your data first.

reinstall firmware?

what's the model of the phone?

> you can try to check for available software updates maybe the software which is currently installed on your phone is too old and cannot support the recent network you are using.
> indicate the model number and ill get back to you on how to do what im trying to suggest.

<Sade>

My phone is very slow, sometimes freezes the phone and you have to take the battery out and replace reeniciar to the telephone. what I do