In a child visitation mediation my son-in-law was refused ability to make a phone call

Mediation is court ordered in Wisconsin anytime there is a custody or placement dispute. Lawyers are not permitted to attend with the parties, and I think it would be unusual for the court appointed mediator to allow any other third parties there or to allow phone calls during the session. Even if the parties reached an agreement at mediation, it is not binding on the parties until the agreement is reduced to a formal stipulation and order and approved by the court. That would allow your son in law after mediation, to discuss the proposal with anyone he wants to, including family, friends, and/or a lawyer. The other beauty about medaition is that if the parties do not reach an agreement, nothing that is discussed at mediation can later be used in a courtroom against him.