The only thing you can change on a Mitel set is the pitch/tone of the ring, but this affects all the times the set will ring, not just for external or internal calls. Could be useful if you had one set in your intercom group that didn't receive 'normal' calls, like what Ralph said.
Program a superkey on a programmable button and this will give you access to 'adjust ringer' then you can adjust pitch and tone.