電話する or 電話をする

I'd like to know if there is any difference between 電話する and 電話をする, if they mean the same, or if 電話をする doesn't even exist.

I ask this because in an exercise in my book it says to use をする with 電話 to form the verb, but on the online dictionary I usually check it says it's 電話する, without を.

Dictionary link: 電話 ? Tangorin Japanese Dictionary

Both are acceptable. You will also find the verb かける commonly used instead of する.