また今度！

I think when Japanese people say "Mata kondo!", it actually means the same "I'll call you!" means in English (or German) - especially between girls and guys, maybe.

Mata kondo また今度 means "some other time" and is used as a phrase when you want to express you'll see someone again some other time or do something some other time.
My textbook (the infamous Minna no Nihongo - このスプーン、素敵ですね！) came with a video where a guy calls a woman and asks for a date, but she's "busy", so she politely declines and says: "Mata kondo onegai shimasu" (Please ask me again some other time.)

Well, I think you can see why I'm writing this. ^^;