The rule is that if he asks to call the other parent and it isn't because he's mad at one of us, we jump to the phone and make the call. I think it's important that he know that either parent will always be there when he has a need.
Fair warning, though I'm sure you don't need it... don't make a big deal about "I miss you" kind of stuff if/when it happens. My stepdaughter turned into quite the little manipulator because her mother thought every "I miss you" was evidence that her daughter was miserable and felt unloved. Actual conversation that took place on the phone at our house:
Stepdaughter: I miss you, Mommy.
Her mother: I miss you too, sweetie, I miss you so much--
Stepdaughter (with a coy little smile on her face): "I want you to cry, mommy."
Her mother: "...What?"
Stepdaughter: "I want you to cry because you miss me."
Her mother: "...Well, uh, I DO, I miss you a lot..."
It was really disturbing. Fortunately she grew out of that phase--now she's manipulative in much more subtle ways.
