Questions & Answers about Позвоните немедленно.
Позвоните is a verb in the imperative form, so it means call! or please call!
More specifically, it comes from the infinitive позвонить (to call / to phone) and is the plural or polite imperative form.
So this form can be used:
- when speaking to more than one person
- when speaking to one person formally or politely
The ending -ите is typical of many plural/formal imperative forms in Russian.
Compare:
- позвони = call! (to one person informally)
- позвоните = call! (to several people, or to one person politely)
This is similar to the difference between informal and polite/plural you in many European languages, even though modern English uses just you for both.