Fax connection keeps MFP477 from powering down

[JonRS]

I have my MFP set up to send and receive faxes via an OBi200 using Google Voice. That is all working fine. The problem I'm having is that when the MFP is connected to the OBi200 via the analog phone line, the MFP never goes into lower power mode where the fan is off. The printer sounds like it's continually doing something (motor noises), and the fan run nearly constantly.

As soon as I disconnect the phone line from the MFP, it enters the lower power mode (I have the timeout set to 1 minute). But when I connect the phone line again, the MFP starts running again making noise.

The MFP is in a home office, so I'd really like to eliminate the noise when it's not actively printing or faxing. Any suggestions on what to do to make it power down when the phone line is connected?

[JonRS]

To be clear, what I'm referring to by low power mode is "sleep mode". I should have used the correct term.

The MFP does not go to sleep when the analog fax line is connected, even though I've got the printer set to go to sleep after one minute of inactivity.