Excuse me? Why does s/he owe you or anyone else "details about his 'impairment'"? My interpretation, which no doubt you will reject, is that s/he is not comfortable making inquiries on the telephone (which, as Doug Miller pointed out, can be worked around by using the various relay services, but which are cumbersome compared with the ease that non-hearing-impaired people use the phone), or s/he may not understand people clearly when talking to them in person. Regardless, the OP is expressing a not uncommon feeling of uncertainty about the information being gathered, especially as a member of a group of people famously disconnected and cut off from the incidental flow of information you (presumably hearing as more than 90% of the populace is) take for granted. Some day you might be in this category, you know, and you'd appreciate a little sensitivity and respect.