I think cell phones are so new we haven't gotten a handle on courtesies involving them. I have seen people interrupt church services with them. I feel that is very rude to be talking to someone and they take a call. The time we spend in the SCA should be  a time where we are escaping from the everyday. How can you escape from the 21th century if you are carrying it around with you? I am sure you could discreetly check your phone regularly if you are worried about something at home, but if you want to get the most out of the SCA it is better to leave the 21 th century at home or in your tent. 
willow