An FXS/FXO does not have setup acknowledge, progress indication, and etc. like ISDN and other signaling protocols. The ringing and connect are in-band, so the gateway will send back that is it connected as soon as it grabs the line.
I would imagine that the same thing happens with a SCCP phone, except that, when using SIP, the call is 'connected' with the H.323 or MGCP-controlled gateway while it is ringing. With SCCP, the phone only receives call control commands from the Call Manager.
That is interesting. I would have expected the callmanager to map the signaling from the MGCP/H323 gateway to the equivalent SCCP and SIP messages.
I was not able to find a solution when I noticed this with an IOS gateway using a SIP trunk to an FXO. Your setup is different.
If you have loop-start FX, "supervisory answer dualtone" in the voice port config may change what is signaled to the callmanager, and possibly ba mapped differently.
Unfortunately, I no longer have any FXO lines in my lab, so I cannot try this myself.
Same here, all this, is for one of my customer and in my lab I don't have what it take to test. My client tell me he already test with the command "supervisory answer dualtone" on the voice port of his MGCP gateway but... I would like to test it myself...