Another reason that I take my phone with me to the restroom is that I'm a paranoid person. What if something legitimately bad happened and I had no way of letting others (DH, parents, police, etc), know where I was. I think this stems from when I was 9 months pregnant and stuck inside the parking garage elevator at work, not once, but twice in a week. The first time I was stuck for an hour with 4 other people and had a full bladder and started to panic. The second time was about 20 minutes and with one other person. The intercom was in poor condition, so I used my phone to call DH who contacted campus police to come and get us out.