indexing on set-length(10) phone numbers for large number of entries?

I understand that a lot of similar questions have already been asked but i could not find a definitive answer for my question.

In very short,

  • I have to authenticate users using a rest api and using their phone numbers.

  • phone numbers are fixed length (10). can be stored in any way suggested.

  • In a table with thousands of phone numbers and the user_id associated, i wish to be able to query and get the user_id associated with the phone number fast.

  • I read that indexing on strings or random integers is bad.

How should i structure the db to query on phone number most effectively?

Thanks in advance!