What Skip said.
If I could, I would try to do it this way:
1] Mount the box first.
2] Have them tile around it.
3] Talk to the tile guy, and convince him not to butt the edges of the tile tightly against the box, instead leaving about 1/16" or 3/32" gap to allow for thermal expansion. This may sound overly cautious, but if a crack develops in the tile right next to the box in the future, the customer is going to blame the phone guy.
4] Have the tile guy apply whatever flexible bead seal around the gap that he recommends. This conversation I would try to have in the presence of the customer, so when the bead dries out in ten years and it needs to be re-caulked, he calls the tile guy and not me.
If I sound overly paranoid, it's because I am. I have seen mirrored glass tiles in up-scale buildings that cost about a hundred bucks per tile, and I sure wouldn't want to get the bill to replace those.
Jim
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Speaking from a secure undisclosed location.