I'm native speaker. "Zadzwoń trochę później" or "Zadzwoń później" sounds more natural. I don't think I'd ever say "do mnie", "I" is implied (at least to me)
Is do REALLY necessary? Ive heard many Poles omit it in conversation.
Ehmm... yes, it is, it makes no grammatical sense without it and I have never heard anyone omit it...
"Zadzwoń później" sounds ok
That one, sure. "Zadzwoń mnie później" (That's what I understand chce_polski was asking about), no ;)