Indexing on a fixed-length string (10 characters - phone numbers) for a large number of entries (performance)?

I understand that a lot of similar questions have already been asked but I could not find a definitive answer for my question. Briefly, - I have to authenticate users using a REST API, with 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 associated **user_id**, I wish to be able to rapidly query the database and obtain the **user_id** associated with a given phone number. - I read that indexing on strings or random integers is bad. How should I structure the db to query on phone number most effectively?