I would guess it's probably your phone's speaker. As the call coming in, the phone fires up the speaker for ringing. The speaker induces a current. GFCI work

I would guess it's probably your phone's speaker. As the call coming in, the phone fires up the speaker for ringing. The speaker induces a current. GFCI works by detecting the difference in the current flowing between the hot wire and the neutral wire. It needs to work very fast (to avoid the human touching the wire getting electrocuted) so any slight difference in current would trip it off. The current induce by your phone's speaker probably causes enough difference in the current to trip it off. You can run an experiment by turning off your phone's speaker for incoming call. See if the speaker is the culprit.