In fact, he has stated his principles/policies publicly and clearly, and someone posted it here as well. He never says "You are not supposed to use someone else's cellphone, but it's okay when I use it." That would make him a hypocrite.
Thus your example is a strawman here, with the implicit assumption that he has said "you ought not to use any cellphone" which he never said.
I think most misunderstandings of RMS come from the fact that people project on him what they think he would say and how he would act, not what he actually says and how he actually acts; while assuming he is merely some sort of activist engaging in civil disobedience and boycott, just trying to make a point and show the world a pure example of how to live. The way he should actually be thought of is he is fighting a war against people building proprietary software that, based on his theory, inevitably will lead to unjust power over users' lives through control over their devices. He is very pragmatic in that sense and willing to take any strategic advantage he can get in that fight, not deprive himself of that just to make a point. My understanding is at the beginning he used proprietary UNIX compilers to build GCC, for example.
Maybe he's not being hypocritical, but that position -- I will pragmatically use others' phones for my convenience, but I will not retain one myself -- just seems shitty. Not sure why, maybe because it's even less of a universally applicable tenet than "phones are bad, mmkay," it's just "I'll use the systems as convenient for me, no matter how others may suffer under them."
I think he makes a pretty big tradeoff that nobody else would, without completely shooting himself in the foot. Even if he had a fully FOSS phone, the people he called probably wouldn't. He's drawn the line far enough out that I don't doubt his sincerity.
I thought I remembered other instances, but none of them come to mind and obviously googling has turned up a ton of opinion pieces without a lot of actual substantative points made, so I'll leave it at that; not enough to argue that he is, by his nature, a hypocrite, but also not 100% pure to his principles in every circumstance.