freezing on checking voicemail

I just bought an iPhone last Friday. I am having trouble when I go to check my voicemail it calls it automaticaly rather than a list the screen goes black and it wont let me bring up the key pad to erase or hang up the call then the phone freezes for several minutes but you can still hear the voicemail menu being repeated. Yes I have it in a case and I have taken it out but still doing this.