There are a bunch of tests to make and if the customer won't give you time after closing to do them then he/she is taking all the responsibility and I would simply say that you can't prove that it is in the 7100 or in the dial tones without his help and I need my Compact back, thanks and bye.
You have no proof it is or is not your equipment at this point and you have no obligation to continue with an obdurate customer.
Time to send him a fax outlining the time it will take to do the tests and stating that without his/her cooperation you will be unable to further service his location.
With four lines this is not someone who has achieved the status of financial genius and you have a right to the time to do your job. Sometimes you have to say, My way or the highway, I have your money and you won't let me make it work.
