HP LaserJet Enterprise 500 Color MFP M575 - Manual (Page 283)

Are you using a phone company voice-messaging service or an answering
machine?
If the rings-to-answer setting for the messaging service is lower than the rings-to-answer setting for the
fax accessory, the messaging service answers the call, and the fax accessory cannot receive faxes. If
the rings-to-answer setting for the fax accessory is lower than that of the messaging service, the fax
accessory answers all calls.
Does your phone line have a call-waiting feature?
If the fax telephone line has an activated call-waiting feature, a call-waiting notice can interrupt a fax
call in progress, which causes a communication error. Ensure that a call-waiting feature is not active on
the fax telephone line.
ENWW Solve fax problems 269