I have Verizon, and have never had much luck on the summit although I've watched others who have.
A fairly reliable signal can sometimes be found at the "windows" located along a half-mile section after Trail Crest where the trail passes close to steep rock walls which seem to focus the signal. Most hikers (me included) are focused on maintaining our footing in these sections (3 or 4 IIRC) but ... if you need a cell connection, find a secure place to stop. One of these windows in particular I've found a strong cell signal.
