Hi, attached you?ll find the config of the voice gw and the ATA186,

I also attached some screen shots and logs I collected with prserv.

Hi, additional the settings of the audio and tone.

Hi all, found the problem. It was an entry in the voip peer to the call manager. The negotiation with the fax was overwriten by the gateway. So, the solution was:

dial-peer voice 100 voip

modem passthrough nse codec g711alaw | the necessary codec

fax rate disable

This causes the GW to comunicate with the codec the fax expects.

The most common reason for faxes to fail in this setup is due to clock slips on the digital interface caused by a clocking mismatch.

You need to add the following config -

network-clock-select 1 BRI0/0/0

network-clock-select 2 BRI0/0/1

network-clock-select 3 BRI0/1/0

network-clock-select 4 BRI0/1/1

Without these commands, the PVDM2 DSP module is being driven by the router's internal oscillator, while the PCM data on the BRI ports is using the clock provided by the service provider. The above commands will synchronise the DSP with the external clock reference (also adds a failover heirachy) and should remove the clock slips which are causing the faxes to fail.