The target is part of new standards set by Ofcom for Openreach, the company that installs and maintains connections to BT's network on behalf of competing providers.
Openreach will have to ensure that most customers wanting a new line receive an appointment within 12 working days. Ofcom said Openreach would face sanctions, which could include fines, it if fails to meet its obligations.
The targets escalate over three years, reaching their full level in April 2016.
Separately, Ofcom will also review the standards of redress that landline and broadband providers offer to consumers when services go wrong.
Ofcom's statement follows BT's announcement that it is to create 1,600 engineering jobs amid moves to improve customer service and install lines and fix faults more quickly.