Here's what my agency says when we get calls from small doctor's offices who don't want to pay for an interpreter:
"There are two reasons to consider paying for an interpreter. The first is legal, the other is business.
Is your office wheelchair accessible? Do you have stairs but no...
Paying the Deaf person directly for the service seems unethical. Different than if they paid $xx for each company or even flat fee for a call.
I bet the company requesting the interpreter would be unhappy if they knew they were paying 5% extra.