This is feedback which, candidly, you
just won't hear that often when you call up office manager Mary at Happy Teeth Dental at the landline number (distributed to several hundred thousand people) which routes to a box on Mary's desk that she is, quite literally, employed to answer. Mary can authorize purchases of thousands of dollars of software. In addition to not being offended by phone calls (Mary is quite good with cutting them short, politely, if she doesn't want to continue them -- c.f. this is her job), Mary can give you very
useful and immediately actionable feedback on them, such as "We couldn't use your application because it doesn't work on the doctor's iPad and that's the only screen he can access in the back of the office." You want to hear that even if you have 0% chance of ever getting a penny from Mary.
If one hypothetically is mortally terrified of sounding like a salesman, you could preface the call with "I don't want your money since $SOFTWARE wasn't right for you, but if you have two minutes, I'd like to hear what went wrong so I can fix it and help our other customers."