why my phone is slow and stop working suddenly?
This likely has to do with your carrier settings. Have you gone to settings > general > about to check for a carrier update?

If you don't see "new carrier settings available" after visiting general, plug your iPhone into iTunes, and manually check for carrier settings.

The next thing would be to go to general > reset and reset your network settings.

If that does not work, back up your phone to iTunes and preform a full restore. This takes a while, but should eliminate most if not all stubborn freeze/slowdown issues.

If it does not, go to settings > general > reset and reset all settings. You won't lose data, but will have to set up your preferences again.