Make sure you have an outgoing number for that extension in the Send CLI Number table since you're using SIP. I believe Declined is error on the display when the field is left blank.

I had one a few weeks back that I had to put in the Group number 800 instead of 9 in the T/S No field in Call Forwarding. Still not sure why. However, that customer had copper lines.

Try turning ISDN Prog Connect On. I know this helps with PRI's but not sure about SIP.