officejet Pro x476/x576 detect dial tone setting missing

I have a laserjet pro 475dn - which I use for fax purposes.

Since we have a phone PBX system - it is crucial that I'm able to disable the "detect dial tone" with this setting disabled - I can send and receive faxes. (without I can only receive faxes)

However we are looking to migrate to the x476/x576 devices - HOWEVER on these units there is no setting that does the above. 

This is also lacking on the 8600 pro+plus models.

Is it really so that there is a limitation to the fax features in these printers? This would be a major blow for our corporation, as fax is a vital part of our communication (unfortunately!)

hope someone can assist.

thanks!