My 9-year old has just started talking to friends on the phone and I'm not sure if you need to worry about time limits, maybe do one phone call and see if you need to? It takes a little while to get used to talking on the phone, although maybe some kids are naturals! My daughter talks for about 10 minutes before she runs out of things to say! Her calls usually don't last that long.
For the first phone call you could call the mom and say that your son wants to talk to her son, and is that okay? If that's okay then I would imagine that your son could call from then on.
Our only rules are she has to ask permission first to use the phone, and we don't allow calls after 8pm. I figure we'll add on rules as we go if needed.