Learn Russian: Можно взять телефон в руку?

The word можно is an impersonal word that means "is it allowed" or "is it possible." It doesn't use a traditional subject like "I" or "you." When you pair можно with an infinitive verb like взять (to take), the sentence simply asks "Is it allowed to take...?" which we naturally translate as "May I take...?".

Why use the perfective verb взять instead of the imperfective брать?

You are asking for permission to perform a single, specific action with a clear result: picking up the phone right now. The perfective aspect (взять) is used for these one-time, completed actions.

Is телефон in the accusative case here? Why didn't the ending change?

Yes, it is in the accusative case because it is the direct object receiving the action of the verb взять. However, for masculine inanimate nouns (non-living things), the accusative form is exactly the same as the nominative (dictionary) form. Therefore, телефон does not change its ending.

Why is it в руку and not в руке?

Taking the phone into your hand involves motion toward a destination. When the preposition в indicates direction, it requires the accusative case, so the feminine noun рука changes to руку. If you were talking about where the phone is already resting (location without motion), you would use the prepositional case: в руке (in the hand).