When I was prompted to enter a phone number I felt a bit trapped because I can't use the phone with comfort and don't want to be contacted that way. But I HAD to enter a number because it is a required field - which is a little ironic seeing this is a site for people who can't hear properly!
How about you have a field where the customer can enter communication notes? If there was one, I would have been happy to let you know that TEXT only on my mobile number was a preferred option - or to email me at my work address.