How would someone that needs sign language assistance let you know over the phone?

People who have hearing problems will almost always have a special phone that displays a running transcript of the conversation. The phone automatically connects to a transcriber/operator when someone picks it up; the transcriber/operator then puts them through to the person they want to call, and starts typing everything that is said.

If they don’t have such a phone, no doubt the person would send a reply saying “Um, I’m deaf. Can you please drop by in person, or arrange an interview instead?”