Generally in the US, if a company is large enough they will provide a TTS number, otherwise you can call via a TTS “gateway”, many of which are provided free of charge by various government entities or by the phone company.

When you call via a TTS gateway, a certified transcriptionist for the deaf or (now) a computer program that does speech recognition will transcribe what is said by the other party to text so you can read it and speak out loud what you type exactly. There’s quite a few regulations around this service, although I’m sure it’s developed since I last looked into it.

There are also special phones or phone apps many deaf people have that do live captioning. CaptionCall is one common brand.