Possibly there is a little bit of confusion on what this entails. Moving from Jabber to Webex is not much more than replacing one application with another. What will change from an infrastructure perspective is that the messaging and presence part will move from on-premises to the cloud. So you can sun set your CUPS infrastructure. Other than that there are no alterations to your existing system landscape. Anything you have now will remain as is.
That is correct. The applications, Jabber or Webex, do not care one bit what protocol you use for your voice gateways. That said, most people have since long made the switch to use SIP as the gateway control protocol, simply because it's so much easier to work with than MGCP.
It's a matter of preference and also other factors that would affect what configuration you'd need to have present in IOS. Like if you use SRST, then you would need to have a set of dial-peers present anyway, so why not go all the way. IMHO SIP is a lot easier to maintain, specifically if you look at it from a troubleshooting perspective. Reading the output for debugs of MGCP is far from easy, where as with SIP it's pretty much all there in clear text.
