My current work requires that I have my phone either muted or turned off, I work on a telephone helpdesk and it is not acceptable to put a person on hold to answer a private phone call. My previous employer (call centre) was the same.
Same goes with other jobs I've worked in including retail, there is no way I would have been allowed to answer a private mobile whilst working, especially when dealing with a customer!
Breaks are for making personal calls. If there is an emergency there is a business number people can call.