Ofcom has fined BT £800,000 for failing to improve a text-to-voice service for customers with hearing or speech difficulties.
The 'Next Generation Text Service' helps users have more natural conversations using speech as well as text via devices such as PCs, laptops, tablets and smartphones.
But BT missed the deadline of mid April last year to launch the service after it encountered technical problems with the sound quality of emergency calls, Ofcom said.
BT launched the service on September 24.
BT told Ofcom that the delay was a one-off incident, and the regulator acknowledged that the problem became apparent late on and the level of financial harm to consumers was limited.
But Ofcom said an improved text relay service was an important requirement designed to ensure that people with hearing or speech impairments had equivalent access to phone services.
It said BT had 18 months to meet the requirement and did not do so for five months after the deadline.
Ofcom's consumer and content group director, Claudio Pollack, said: "The size of the penalty imposed on BT reflects the importance of providing an improved text relay service to its customers with hearing and speech impairments.
"However, BT has invested significantly in launching the new text relay service, which allows users to have conversations more easily and fluently and on new devices.
"We welcome the fact the service is now operating successfully."
A BT spokesman said: "We're sorry we had to postpone the full launch of the Next Generation Text service. This was because of a safety issue with the quality of emergency calls that could have put users at risk.
"We fixed the issue as quickly as possible and, after fully testing the service, launched it at the beginning of October 2014.
"The service has been warmly welcomed by users. Hearing and speech-impaired people can now make faster, more fluent phone calls using ordinary smartphones, tablets, laptops and PCs, as well as existing specialised terminals."