create custom caller id

I am trying to figure out how to set caller ID on follow me calls or ring groups.

What we have is an emergency service for our clients. When they call in the IVR allows them to select an option for urgent support. If they select this a ring group is called. This ring group is a combination of internal and external numbers. For the internal numbers I can see how to set a CID prefix, but for external I would like to transmit either the caller's ID or simply a specific ID of our choosing - such as an extra DID we have. I am not absolutely committed to doing this as a ring group. If it is easier to do as part of a follow me I could use that as well.

Thanks