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Fax from MGCP VG224 to SIP

Level 1
Level 1


I am trying to fax from a VG224 configured as MGCP. I have a 2821 configured as a H323 CUBE. A Verizon SIP trunk is connected to the 2821. So the fax flow is this:

VG224-->H323 CUBE-->SIP-->SIP provider.

I can't make it work since the fax call will never negotiate G711 codec. Has anyone ever make this work? It works if I configure the Vg224 as h323 but this is a management nightmare with all the dial peers as we have a lot of fax machines. If anyone has made faxing from MGCP thru SIP work I would appreciate some config samples. Thank you.

Vince Doan

4 Replies 4

Hi Vince,

You can disregard your other post, the other post makes it appear that you have H323 directly from your VG224 to CUBE which isn't the case here.

This is your current predicament:


Verizon as a SIP provider only supports fax pass-through. They do not support any NSE methods, and they do not support T38.


SIP on CUBE supports all methods.

H323 on CUBE supports all methods.


MGCP supports NSE based passthrough and T38, as well as protocol based T38. It does not support pass-through.

In short, Verizon is only going to support G.711 pass-through, and that's not supported on MGCP (or SCCP).

I would suggest switching your VG224 to H323. It's possible to do this with a minimal dial plan. Point all calls from your VG224 to CUCM, and make a dial peer with the particular destination pattern to the VG224.

Once your VG224 is H323:

voice service voip

fax protocol pass-through g711ulaw

Use that on both your VG224 and CUBE.

When Verizon doesn't support T38 it puts a damper on faxing, especially when you're using MGCP.



Hi Nick,

thank you for the reply. So suppose that verizon supports t38 then this should work with MGCP, correct?



Hi Vince,

Correct. From my experience, they don't.


Thanks Nick. Looks like I will have to use H323 all the way from the VG224. More management overhead but it works. Thank you for you help.