Can you post again with a bit more information? Things like whether it's just started, whether anything has changed (device, software, apps), whether you can make outgoing calls, whether it's some or all calls not coming through, whether the same is true if you use a different handset (assuming GSM, so you can swap SIMs), whether it depends on location, etc? And what happens to incoming calls (dead line, divert to voicemail or some other number)?
This will help us identify whether it's a network/account or a phone problem, if phone whether it is software or settings.
It may be that we need to ask questions back and forth to work this out. If so, that would work more effectively if you registered with the forum, since you can't reply to this mail using the guest questions interface, just post a new question, and more people will see it in the main forum.
Good luck!