。
I'll call you tomorrow morning.
Literal
Tomorrow [possessive-の] morning phone-do.
Maximally compressed: no subject, no object, no particle for 明日の朝 (it's a time expression, which doesn't take に when relative to 'now'). 電話する is the する-verb form of 電話 ('phone'), standard for 'make a phone call.' Both the caller (speaker) and the receiver (listener) are implicit.