I agree with Ralph. Ensure that these settings are correct. Also if you are using a wireless IP Device that is Mitel, you should not need to create a speedcall, you should be able to put the extension in 'Call Reroute 1st Alt'.
another way of doing this is a bit long but does do what you want it to.
1. Create the key system number as a device on the system
2. Registure that device to a IP phone.
3. Set the call forward on no answer (internal/external) to the destination number
4. Keep the number on the system and remove the mac address
5. Program this number on the phone you need this on.
If you have an opsman, just do the programming from there.
This always worked a treat when you have different systems connected together, such as Mitel and Cisco
I hope this helps