He needs to take the stick out of his ass, especially if it was his personal phone and he was alone in the office. Was this the first sexy text message you've ever sent him? Does he act like this whenever you bring up something sexual to "discuss?" Have you sent him dirty pics on his phone?
I mean, my husband and I both have Gmail accounts, and when I first started sending him dirty messages (in the first sentence), he got a little paranoid because he worked in a quad-cubicle where people might glance at his screen over his shoulder... and if they saw the first line, they might get excited.
But he just told me to place sexual messages later on in the e-mail, so no big adjustment there. Then again, we both talk about sex very openly and used to send each other dirty pics on the phone as well (when our phones had cameras). So, that may be the difference here, I don't know.