Another one is http://www.maxmind.com/app/telephone_overview

This is a classy company, very reliable. We've been using them successfully for years for fraud prevention in online credit card charging.

How does phone verification work with customers who are deaf? Does the API could provide an option where the computer calling the customer plays Baudot code [http://en.wikipedia.org/wiki/Baudot_code] instead of speaking the numbers in English?

Text messages could work in that case, no?

Here. here. We use MaxMind.com for their GeoIP product too