Once you drop your appointee off - you always answer the phone. DS got a phone call after glider ride during BCT, but many also got phone calls after they were at the firing range. After BCT was over he didn't have his phone til PW, but his roommate had it one weekend for a family emergency situation and was "allowed" to let his roomies use it discretely.
If your child is an IC - the upperclass IC's will hand their phones over all the time.
Bottom line -- answer all phone calls at anytime of the day or night, from now until they separate/retire from the AF.