First things first, is that working fine (is that call ending up in VM)?
Once you confirm that the rule is working for you, I would try this to give him a busy tone, choose to forward to SIP URI, enter a non-existant SIP URI, like
111@sipbroker.com....
Curious, where is this call from (what country/region) for it to send a 8 digit CID (depending on where the number is based, it may be possible to spoof the CID so you can test how the rule is behaving without having to wait for these guys to call again)