Setting different ringtones for incoming calls and Desk-to-Desk calls globally?

My office recently upgraded our phone system and we love the new system except the only issue is most of the staff has the VVX 300 series and cannot tell if someone is ringing them desk to desk or they are receiving an external call from outside clients/customers. Is there a way to change the ringer for external calls vs. intercom calls? Globally would be better, but even individually would be fine.