You mean dial tone, not ring tone.
A phone port has no automatic logical connection to any 'line' (the POTS port or a SP or Obitalk). Thus it does not in general make sense to have a special dial tone when a line is forwarded.
Nevertheless if you want a reminder that forwarding is set, study the section of the Admin Guide on Star Codes. A Star Code can perform more than one action, so in addition to the call forwarding, insert an action to enable Stutter Tone.
Similarly for the companion Star Code to disable both the forwarding and the stutter.