When they call, accept their invitation and wait 10 sec. Call them back again using the "cancel plans" option. They'll be ok with it, no harm no foul or decrease in likeness.
Also, in your phone, go under options, and turn phone to sleep mode. It blocks all calls until you activate a mission. Useful when you want to cause some mayhem or collect stuff. I hated it before when I was in a shootout with SWAT, and Roman calls asking if I wanted to go drinking....
Oh, and if you are on a mission and your friends call, you'll automatically reject them, but with no decrease in likeness
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