While on line, how can incoming calls be forwarded to my voice mail?

GE Cordless, Model # 25839GE3-C S/N: 50420375
When an incoming call comes while I am speaking on the telephone, a weird, awful noise is heard by me (not the party to whom I am speaking). I know that someone is trying to reach me. How can these calls be automatically forwarded to my voice mail without interfering with my conversation when I am using the telephone?