I guess some people might get mad, but I'd actually be more mad if my boyfriend didn't answer my phone. It could have been an important call. I also answer his phone and he doesn't care. I'd imagine that a guy would only be mad if someone else answered his phone because he was hiding something and was paranoid about her finding out. If it was a work phone, then I'd understand, but me picking up the phone when my boyfriend's mom calls isn't going to do any harm lmao.
I'm not actually dating anyone at the moment, but if i was and if i had to leave a room or anywhere else, then i would give her my phone I case it rang. Also I have nothing on there to hide. It's no bother to me if she goes through it during my absence.
I can honestly say for the most part no. My girlfriend and I share a female friend though. Our best friends we double date with a lot, if I was busy & my friend answered the phone my girlfriend would be ok with it. Only because she knows her & she's equally as much my friend as she is her's if not more so. If it was one of my friends from school oh god... I might as well grab a shovel dig a hole & knock myself out it be less painful.