Motorola V620

I've had my V620 since the beginning of April. I've had no problems until now. My voicemail. I'm on Cingular GSM network. On Saturday April 16th, I tried to check my voicemail and I got a busy tone. I don't know why. Friday I checked my voicemail but on saturday it was busy. My sister has a V180 so I switched sim cards and put my card in her V180 and it allowed me to access my voicemail. I thought maybe it was the service but it's the phone! So if anyone can help me out and show me how to check my voicemail with my V620 that would be great. It worked before but now when I call I get a busy signal. Thanks