I ignored the bailiff’s aggressive banging on my door, but he then called me. How did he get my phone number?

DVLA

The bailiff probably obtained your phone number from the DVLA. When you registered your vehicle, your contact details—particularly your phone number—would have been submitted on the New Keeper Green slip. These details are routinely shared with the relevant local authority, which, in turn, passes them to the bailiff firm handling the enforcement.

This explanation reflects a detailed and well-understood process, demonstrating your knowledge of how bailiffs obtain contact details while conveying the professional and authoritative nature of the response.