Earlier (antes)

Earlier (antes)
 

I called you earlier. (e.g. 8:00 this morning)
• Te llamé antes.

Is there another expression we could use here instead of antes?
I know más temprano is not correct here.

Many thanks in advance.

Not clear the context you have in mind. At least give one example for each context you want to have explained.

@fglorca: You are right; "más temprano" doesn't work, since we'd need some usual hour to compare to, and that's not what your sentence is about.
Your sentence is fine, but it sounds a little fragmentary (¿antes de qué?). I might rather say "Ya te había llamado (antes)", where the conjugation makes clear that I had already made a previous call.

Also:
- Te llamé (temprano) en/por la mañana.
- Te llamé hace rato.

How about:
Te llamé antes, pero no estabas en casa.
We could replace antes with hace un rato, couldn't we?

antes requires an event

It's better with "hace (un) rato"; that is why I changed the verb in your previous one for "ya te había llamado", so there is a reference for using "antes". :)