If you're leaving and a few minutes passes and you come back..fine. I don't constitute that as rejoining to gain an advantage. BUT, if you were to continue doing this, than it may become an issue of your reasoning being questioned. Anyone can say they get work calls if it helps them to leave and rejoin constantly.

Most admins are willing to listen if you say you had to take a work call..just don't do it a lot, or the admins may not buy it.