It's possible that this person is in a position like that and is on call for work, but it still feels somewhat disrespectful to those who are training with them.
Heck yes, if they didn't bother to explain the circumstances. OTOH, if this happened at my dojo...well, I can't conceive of anyone doing it under any other circumstances, and they'd definitely have it on silent (so they would have to check). Do you think it's still disrespectful? Do you think that on-call people should simply not train?
We also have some people who often come late to class because of their work/family situations. This is tolerated because it's understood that they're doing their best to get there as soon as they can (and are, in fact, working harder to get to the dojo than the rest of us). I know that at some dojos, this simply wouldn't be tolerated for any reason. Do others feel that this is disrespectful?