[Freeswitch-users] <condition/> and custom functions in lua or p-languages?

We need to write a function, usable in the XML dial plan, that tests the inbound CID (not destination_number) against a "white list." Something on the line of,,, <condition field="ISWHITE(${caller_id_number})" expression="true" > Question: In such a use case, must ISWHITE() be a custom application/mod written in C, per skel.c, or can it be a script written in lua or p-language or the like? It's clear from wiki and docs we've seen that <action ... /> can have lua or p-language scripts; but it is not clear about the <condition ... /> clauses and where to locate and "hook" them in. Guidance and any pointers to docs/examples greatly appreciated. With thanks, /S