There is a service called Text Relay. It's where there is a 3 way conversion with yourself, a customer and a third party member, who types everything you are saying, to the customer, who may be hard of hearing or deaf. This is a good website to explain it: https://www.relayuk.bt.com/
There is a company called Relay UK, reachable on 0870 240 5152 that provides a free service to use, when needing to call a deaf person. If you call their number first, it will then ask you to input the customer's number. Wait for a member of staff to pick up and then they will call the customer, to let them know they are also on the line. Then speak slowly to the customer, as the Relay UK staff member will be transcribing what you are saying to them via a typing machine.