Sorry to play 20 questions, but could you please answer the following:
What you call your number from someone else's phone, is the call simply declined or does it ring?
Can you send and recieve texts?
Can you use the internet?
Let's try this:
1) Turn the phone off for 10 minutes before turning it back on again.
2) Try calling or recieving a call. If no cigar, proceed.
3) Remove the SIM card for 10 minutes before reinserting it.
4) Try calling or recieving a call. Let us know the results.