Primarily the service was set up for the deaf or hard of hearing but then rolled out more extensively
“When you send a Deaf 999 SMS, all the text following 999 will be converted to audio. The service will then dial the 999 emergency service automatically. When connected it will play your audio to the emergency services operator.
You’ll receive response texts back to your mobile when:
A: The operator answers the call.
B: When the operator selects a response option.
C: When the operator hangs up the call.
The text number 50123 is a freetext number. Texts can be sent to this number even if a mobile is out of call credit.
The operator will always be informed of your mobile number and any other network information related to your call.”
http://voicetexts.com/PhoneSettings.html