Understanding multiple indexes with MySQL (and performance)
You speak of "the index" so I assume you mean "the clustered index."  I'd actually start by just slapping a unique index on phone_number - if you're confident that it really is and always will be unique, and if it's the field you most often filter by.

If it is unique, it's a good example of a "candidate key" - you could, technically, use it as a primary key.  However, it's also a "business key" - driven by human logic, and therefore susceptible to change, unlike an IDENTITY or other machine-generated sequence, like your phone_id field.  Biz keys are generally poor candidates for the PK, thus sticking with your current PK/clustered key.