Samsung Galaxy Ace GT-S5830 can't receive incoming calls
Under "Settings" then "call settings" in gingerbread there are two settings that might be doing this. 1 call forwarding. this might be referring incoming calls to a dead number. 2 Call rejection. This could be rejecting your calls - just adjust the options set.

If you have a new phones but amended its 'birth' number to the original number you used on the old network this might have something to do with it too. I did once have a phone which in certain situations remembered its original number which was by then not live on any network- just a thought.