I'm pretty sure I'm misunderstanding something, so bear with me. An external caller would need to have dialed something to ring your PBX...a DID.
You should then be able to direct that DID to ring a particular phone or group of phones without issue.
A NON-DID, by definition, does not receive calls from outside the PBX.
If you have a SIP circuit and a Mitel Border Gateway, I do believe you could set a rule whereby a specific incoming calling party number would route to a particular controller....though the only thing I have ever set up is a sort of "Black list" where I send those specific incoming numbers to a controller that doesn't exist, but I imagine the reverse is certainly possible.