Dear Howdyhar
Yah, it does rather come off as fishy. Sorry to say but that's why your here right? A little brute insight.
Many of us would get the same impression. He's turning off his phone so he doesn't get a call from you while he's with so and so. Again though, this isn't conclusive.
He could very well simply n e e d a time out from the phone. Some hint to this would be say, his cel phone is constantly demanding his attention when he works and since he's on vacation, screw that type thing.
It all depends on how you two do together and only you two can know this.
Why are you not going with him on this vacation? Is this a long distance relationship?
Follow your gut instincts and see how often he calls you from his vacation; might give you more to go from; but hey, don't be this guys booty call.