If you give me the full sentences, it would be better. The first one "전화해 드릴게요 " depends on the sentence, it could be "I will make a phone call instead of you.". If there is "나중에" before the phrase, it means "I will call you later." "전화할게요" also depends on the sentence. For instance, let's say you and me are in the same place, and you say "전화할게요". In that case, it means "I will call someone". However, if there is "나중에" before the phrase, it means "I will call you" (but it's more casual than "전화 해드릴게요".)

전화해 드릴게요 and 전화할게요 They are the same as far as the meaning goes, but the former is more polite and formal. Also, 전화 드릴게요 is more common than 전화해 드릴게요, although they have the same meaning.