Although a better place for this might be on the extensions setup page, it might be easier to implement in Follow-Me because there is already a provision for setting the Alert Info field there. The basic idea is to split the current field into two fields, one that contains an Alert Info string that would be sent if the call is "internal" (comes from another extension on the system) and the other contains an Alert Info string that would be sent if the call is "external" (comes from an inbound route, or otherwise a number external to the system). Whether a call is classified as internal or external would be based solely on the contents of the ${CALLERID(num)} variable - if it matches a known internal extension, then the call is internal, otherwise it's external.
(Granted, it would be possible for a more complete implementation to allow yet a different ring for a "transferred" call, but that could not be easily determined by comparing the contents of ${CALLERID(num)} to the known list of extensions and other "internal" numbers.)
By the way, I tried to work out a way to achieve this functionality using a bit of code in extensions_custom.conf but ran into a slight snag, see http://www.freepbx.org/forum/freepbx/users/how-to-make-internal-calls-ring-differently-from-external-ones for details. But it would be much easier for users to have the split Alert Info fields on the Follow-Me pages.
