Call forwarding on an SL2100 off premise gives a busy tone.

Forwarding to an internal extension works. Is there a settings that needs enabling to do this, other than setting the trunks to allow trunk to trunk transder and off premise forwarding?

Look in hold/transfer settings under COS.

That's all enabled. could it be anywhere else?

So 20-11-12 is enabled? What number are you forwarding to? Did you remember to put your trunk access code before the number? Do you have more than one trunk?

Yes this option is enabled. I have 4 trunks in total, and yes, I am putting my trunk access code into the forwarded number.

Is the station in the correct COS? Does the call make it out of the system? Does it actually grab another line and try to call out?

No, I think that is the issue, it doesn't attempt the off-premise call. It's on a VE that I have the forward setup, although I have tried it from an extension too which also does not work.

If you are not able to dial 9 from a working station indicates to me more programming needs to be done by the installer.

Sorry no, just to confirm, the system is functioning and wokring normally. Everything works correcty aside from the fact you cannot call forward to an off-premise number.Ony call forward to internal extension works.

We can make outbound calls to the numbers without any issues.

Have you checked your Trunk Group Priority ordered number list under 14-05? If they are still set to the default settings and include Trunks in the ordered number list that are not installed on the system, you will have issues (such as busy signals/disconnects) when you attempt to do call forwarding off-premise. This would particularly an issue if higher priority numbers such as 1 or 2 are assigned to non-existent trunks.