This is what I do.
I created a contact just for wrong numbers / solicitors. Every time I get an annoying call, I add that number to that contact.
Through contact management on Android, I set all calls from that contact to go straight to voicemail. And through Google Voice, I set up that contact to be blocked completely. And the only number I ever give out is my Google Voice number.
So chances are they'll be blocked completely. But just in case they get my T-Mobile number, they'll go straight to voicemail.