In 9006, SYSDA allows you to change the ringing parameters, but there does not appear to be an entry for the busy signal - otherwhise you could just change the tone, durations and breaks. You could also look at TIMC and TIMD1 where some of the timers are set, but be careful - you can screw yourself up pretty badly on a systemwide basis in there.
Hicom 150 has a whole screen of timing parameters you can mess with.