A bit more information would be helpful.
What version 3CX are you running?
Did this issue just start, or has it been happening since the install? If it just started, what changed on your network at that time?
Are there any associated logs in the 3CX activity log when registrations fail?
Do all services show as running before you re-start?
Internal calls are unaffected?
VoIP provider registrations are still working?
Have you determined what time of day this happens, is there a pattern?