Anything that is interesting to you, is interesting to her. Let that be your state of mind before you call her.
You just have to watch your tonality. For example: "Oh man, you won't believe what happened to me today!", might be more fun to listen to then: "Hey, today I did this X, which was very fun."
What did you talk about with your gf for 5 years? I'm sure you know what women like to talk about.
If you run out of things to say...
I have this one routine, which is actually multiple routines combined I found on internet, I call it the Piercing routine. It contains DHV spikes (Pre-Selection) and flirty stuff to talk about. It's also great for kino escalation.
1. "Hey btw, give me your opinion on something. A friend of me told me I should pierce my ear...she was all like 'yea that would be so sexy on you'. Even though I'm not sure yet if I should take it, which ear should I let get pierced?" (I believe right ear is gay, but doesnt really matter, if she doesnt know u just say its so, and start the discussion on it.) "So if someone, like you for instance, has both ears pierced, does that make them bi?"
2. Tell her about how u dated a girl who was bi for a couple of weeks, and she kissed with another girl on a party once. Ask her if that's cheating. You agree with her, w/e she says. (yea, friends told me it was wrong, but since it wasn't a guy, it was no big deal for me / yea, it kinda upset me as well, so I broke it off).
3. Who cheats more often? if she says guys --> you had some bad experiences with men? And u didnt cheat at all in your life? If she says girls--> oh...so u cheat alot? Accuse her of being a player and only want men for their bodies. "And i figured you for a sweet girl."
4. Who lies more. Often i get girls as answer --> oh, so you are a good liar? I bet u suck at it

. I have a way to test it.
5. 5 questions game.
Since you are calling her, u can bet her she will fail, and to buy you a drink this friday at 9, at X. Otherwise, you will buy her a cocktail at Y.
I never did the 5 questions game on the phone, but I think it will work just the same.