Customers → Customers.
Each record holds contact details, licence details, and every booking that person has had with you.
Matching is by email
A returning customer is matched on their email address. Book them under a different address and you get a second record with none of the first one's history.
So when you take a phone booking, use the email they booked with last time if you can. It is the difference between "your fourth hire with us" and a stranger.
Duplicates
If you do end up with two records for one person, the giveaway is a customer whose history looks shorter than you remember. Get in touch if you need them merged.