Again, I think your setting a contradictory standard by saying that divorce should only be allowed in case of adultery and then define adultery as mere phone sex or masturbation. I can just imagine the divorce courts if such a rule were adopted. Wildcat wiretaps, spoogey tissues, where would it end?
It is the preferred solution for the wronged spouse to forgive.
It always is. That's the Christian thing to do.
Biblically the husband's body belongs to the wife and the wife's body belongs to the husband. ANY sexual activity outside of that is adultery. Both phone sex and masturbation are sinful in their own right and both fall under the realm of adultery since they deprive the spouse of what is rightfully theirs.