"Call me tomorrow."
Danieleさんこそ、日本語がとても上手です。 Yes, my language is Japanese. So I am bad at English. I write more a bit. The order of words is important in English. You know the order in Japanese sentence is flexible than English. The particles are more important instead of the order. 明日私に電話しなさい。 This sentence can change 私に明日電話しなさい。 It is important that you make 私 and に to move together.
There are many cases, I said about 'this' sentence now.
I don't know which is close '電話してください' or '電話しなさい' as 'call me'.
But I think that these are different. '〜しなさい' is the command. Mother says to her child. '勉強しなさい' and '部屋を掃除しなさい' etc. The mother does not need the answer of child. The child have to do.
'〜してください' is request. This is polite than 'しなさい'.
And it is good you said '電話してください' than '電話しなさい'.