Generally it's not so much about what company versus how many people around you are using it. It happens pretty often at the Metrodome; several friends of mine were with me at the Twins last 3 home games/the playoffs and none of us had any luck getting any sort of cell coverage (despite allegedly having full bars). Three on Verizon, two on AT&T 1 each for Sprint and T-Mobile. Same deal for Vikings game. I can't speak for TCF, since I never have my phone on me for band, but I assume it's more of the towers getting blitzed by thousands of users at a signal point in time.