Where do you wish to add ringback? When UCCX is ringing the Agent phone?
I would advise against this. At first it may seem like a good idea: it lets the caller know they are about to be connected to an Agent.
However, as soon as that Agent fails to answer: for training, technical difficulties or other reasons, the caller is going to be confused and your solution is going to appear broken.
If you are still set on doing this, then you will first need to configure a new MOH Audio Source in your CUCM which pays a ringback.wav of your choosing. Then you will assign this new MOH Source to your UCCX Call Control Groups as the Network Hold Audio source.
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