Hello Oscar welcome to the forum,
Can you please send me a private message with the customer in questions address and number so we can investigate?
If you have Polycom phones this should work for you.
Here is what I did.
You will need a Bogen Universal Telephone Interface (UTI1) about $160 buck on Amazon.
Have Comcast set a extention to ring to an ATA. From the ATA (port 1 on my Cicso) run the phone cord to the UTI "Station Port".
Follow the instruction on the PDF linked below.