Depends on the PRI carrier, and what they allow to be sent as outgoing CID. If they allow numbers other than the Main number to be sent, at least DIDs that are part of the circuit, and the DID ends with the extension number, you could create a new ARS table with N; / Dial3K1 / NS2223334E / PRI Line Group, and a Short Code like 8N / Dial / N / ARS for Twinning.
User would program their Twinning number with 8 and cell phone number instead of normal 9 and number, example is 3 digit extension numbers matching last 3 of the DID.