Generally speaking, I'd not get involved, until the phone call was ended and then I'd politely ask if everything was okay. If it was someone who I knew they constantly talked about petty personal smack to, I'd not bother with them until they felt like opening about it. If I could sense this was a much darker issue, I'd still stay out of it and subtly say later they seemed upset and if they needed me for something or to listen to them, I'd be around.
I'd leave them alone, simply because I know that's what I'd prefer people to do if it was me crying. I'd maybe go by later when they'd calmed down a bit and ask if everything was ok. But often people get embarassed to be seen in a state, and unless you are a someone they would talk to about emotional things anyway, they won't want to talk about it until they've calmed down a bit.