I have a problem with my blackjack 2 for ATT , most off the time i dont receive incoming calls, can som1 help?

Hello, I have a technical question and would be glad if someone can share some knoledge. I've got problem with a Samsung blackjak 2, that doesn't receive incoming calls properly. Approximately most of the time we attempt to call my cell phone its ringing but i dont recevie the call as if i was not answering my phone and it doesent show dat i have missed calls or anything. What I concluded after some experimenting is that the device receives calls only wen im making a call , but some of the times its very slow in connection when im trying to reach someone and sometimes the calls jus disconnect. The phone is in a area with good reception and doesn't seem to be influenced by caller, network load, surrounding objects etc. Do you think it is a hardware problem or it may be software as well, or is it better if i jus get a new sim card?