I sometimes call my gf and tell her I'm gonna do something. Then I mute my side of the call so that I can listen to her sing.
Why would you do that to your gf?
I just wanted to hear her sing very badly.
It's really kind that you are so interested in hearing your girlfriend sing, but next time perhaps you should just ask her to sing for you instead of lying about the situation.
I'm going to continue doing it if she says so.
While it is cute that you like to listen to her sing, it is not a good idea to get into the routine of lying to her. If you do not feel that she is a good singer than you should either tell her or simply stop lying about doing something.