If you are installing the imp on a line with a fax machine, you will want to install the imp between the fax and the phone line to avoid blocking incoming faxes. You may lose some Caller ID information, however.
Alternatively, you can try installing the imp before the fax machine, but you will need to turn on “Allow All Calls” when you are expecting a fax.
We do not test the imps compatibility with fax machines, and there may be instances where imp is simply not compatible with your fax system.