Here's why:
* Imperative sentences give commands or instructions. This sentence is telling someone to call their mother.
* They often begin with a verb. "Call" is the verb that starts the sentence.
* They usually don't have a subject. The subject (the person who should call) is implied.
It's important to note that while the sentence is grammatically correct, it could be written more clearly as: "Call your mother as soon as you arrive in Canada."