Sometimes you can program this into the dialplan of the IP Phone or ATA adapter you are using, and this can be much easier than trying to come up with a round-about method in Axon.
For example, almost any Linksys/Sipura ata adapter (e.g. the PAP2T) will have have a built-in dialplan option that gives you high levels of control over what numbers can be dialed, how they can be dialed, and how they should be processed before sending them to the SIP server (Axon). You could for example restrict calling to some numbers but not others (e.g. everything but 911), or you could make it a 'hot-line' phone, or just prevent any outbound dialing at all.