To use the service, a caller lists the emergency service they require, followed by a description of the problem and the location in a text. It has also been used by those with allergies who are left temporarily unable to speak due to anaphylactic shock. A spokesman for BT said that it “did not recommend” that people without a disability use the service. He added that if a caller is unable to speak, the “Silent Solutions” rule is a better way to communicate with emergency staff. This means that a silent 999 caller can let operators know that they are experiencing a real emergency by responding to prompts to cough or tap the handset, or by pressing “55” on their keypad. Want the best of The Telegraph direct to your email and WhatsApp? Sign up to our free twice-daily Front Page newsletter and new audio briefings. Callers have been warned against using a text service designed for deaf people to contact the emergency services.According to BT, 250,000 people are registered with a text service which allows users to contact emergency services without speaking on the phone.The service, which was set up to help deaf people and those with a speech impediment, could allow those in a hostage situation in which it would be too dangerous to speak to call for help silently. But both BT and Ofcom have warned that the system should only be used when necessary as an increase in demand could cause capacity issues for disabled users.It also has a slower response time than a 999 call.A spokesman for the communications regulator, which requires mobile phone companies to provide the service, said that it “has been designed specifically for people with hearing loss or difficulty with speech”. Users register for the system in advance by sending the word “register” in a text to 999 and replying “yes” to the response, which will include information about the service. BT does not record whether those who register are hearing-impaired or not. Currently the service receives only 14 requests a week. The system, which has been mandatory since 2011, relies on “relay assistants” who dictate the contents of the message to a 999 adviser and write down the response to be sent back to the caller in another text message.