てもいいですか vs. causative verb + もらってもいいですか

Question

電話を使わせてもらってもいいですか。

I know that causative verb form + てもらう means to let somebody do something. And てもいいですか　is for asking for permission. But I don't quite understand the use of these 2 constructions together.

If I wanted to ask someone to let me use their phone, I would possibly say:

電話を使ってもいいですか。

How are the meanings of the two sentences different? Is "電話を使わせてもらってもいいですか" just more polite?

Answer 1

てもいいです - May I do(To Grant Permission)

causative form + もらってもいいです - Would you Please let me do(To seek permission)

Answer 2

電話を使ってもいいですか。

literally means

Is it ok for me to use the phone?

And

電話を使わせて貰ってもいいですか。

literally means

Is it ok for me to be let to use the phone by you?

The latter one sounds more polite.