For places that require a phone call I came up with a trick to cancel via email. If they reply back via email and say I have to call them, I tell them I'm deaf and can't talk on the phone. Works every time.
But sadly there have been more times than I can count where I told them I have a speech impairment, it’s difficult for me to say certain things in the way they need me to right now, and I’d prefer to send them an email (even offer to send them an email for them to open while we are still talking)... And they said no they cannot do that for one reason or another.