。
I'll call you when I get back tomorrow.
Literal
Tomorrow return-[when-たら] phone-do [polite-ます].
~たら here is the temporal conditional — 'when X happens, then Y.' Unlike ~ば (which is more general/hypothetical) or ~と (which expresses automatic results), ~たら is the most natural choice for sequenced future actions tied to a specific event. 電話する is the verbed form of 電話 ('telephone') — する verbs are how Japanese turns nouns into verbs.