I've been with Verizon for 15 years and before that, Cellular One.
I currently have T-Mobile, and I travel for work. Hasn't been an issue yet. Since everyone is going on about longevity, I've only had cell phones for 8 years. Started on Virgin, moved up to Cricket, to T-Mobile, to Verizon, to AT&T, back to T-Mobile. Verizon was the most stable.