IPKall, Voipdiscount or hardware problem?

I have not been able to received phone call or dial out since Saturday. I use IPKall number and Voipdiscount for service. When I tried to dial, all I get is busy signal. I have been have voip for almost two years and never had problem. I wonder if it is a server problem or modem problem. Has anybody run into this? I don't know where should I start to try to get this fixed. Thanks for your help!