iphone 4 getting no service almost daily

so my mom got my sisters old iphone 4 (on ios6) and everything works great except it gets a no service pretty much every day.

tried another sim card and its the same. a reboot will "fix" it until next time.

most of threads ive found mention dropped calls etc but nothing like that has happened here (perhaps she has just been lucky).

anyone have any idea on what the issue might be and how to fix it?

thanks in advance