In a custody/divorce case is it allowed for a plaintiff to contact the defendants attorney to discuss the case via telephone?

If the plaintiff is representing himself/herself, then the plaintiff is actually his or her own counsel.  At that point, yes, a plaintiff, who is pro se, may contact the defendant's attorney to discuss the case.   However, if the plaintiff is represented by counsel, contact with the other party's attorney is entirely improper.   I hope this helps.