I have had to address this. I have asked them if there was some kind of medical issue as the other employee feel they are being left to do the work on a consistan basis. I have told them if the condition inhibits their abillity to work at certain times we need to address it.
If their is no health issue, I also have a cell phone policy. If I believe they are using the restroom to talk on the phone, I tell them to leave their phone on the counter.