The logical thing we can do here is to look at the new gateway that is processing the call. Since this parameter changes the gateway used to route the call.
Have you looked at logs/debug on that gateway of the called party?
If the call doesnt hit the gateway then we need to back up to cucm to see cucm logs.
A call from IP phone to another extension (extension dialling - 3 digits) that has call forward all to mobile gets a fast busy tone.
I checked on the gaateway and the call doesn't reach the gateway, it's blocked at CUCM side.
Please keep in mind that the call from Jabber works fine.
In that case we need to look at CUCM logs. Can you provde that?
I understamd that jabber works, but that is only half the picture. We cant speculate, we need to trouleshoot..
What dial plan apporach are you using, PSTN on phone and blocking on line? etc
Are these users also using extension mobility? Found this can also have an impact and think there is a bug with EM and SLRG, as ran into it ages ago, not sure of the specific CM version.
I have found that it is more reliable to create a specifc CFWD CSS, but always looking into other methods I personally need to do a bit more research on this myself.
Please provide detailed CCM traces from all Call Manager nodes with the numbers that were dialled and time stamp.
If your description is right, this might be a bug and you need to involve TAC. From what I remember, this is a known issue. Let's see the traces.