。
Don't forget to give me a call tonight.
Literal
Not-forget-[without-ず] night honorable-phone [request-ください]
忘れずに is a fixed adverbial built from 忘れる + ~ずに ('without forgetting'); it functions as 'don't forget to X'. お電話ください uses the honorific お- prefix + noun + ください pattern — a very common polite request structure in Japanese, slightly more deferential than 電話してください.