From FreePBX Community user w5waf
Try creating a couple of custom extensions -- 772 to start call forward, 773 to end call forward.
In the dial field, for 772 try something like ZAP/g0/*721NPANXXXXXX and for 773, *731NPANXXXXXX.
I've got an older box with "ZAP" channels, so you'd use the appropriate dial string for DAHDI. I think it is the same replacing DADHI for ZAP.
If you're unsure what the dial string is, you can go to the asterisk command line and watch the log output as you dial a number. You'll see the syntax to use.
It MIGHT be necessary to put a "w" (wait) command or two in the dial string after the 72 and 73.
Once you have this working you may use the 772 and the 773 extensions in your dial plan, like setting up a "Day/Night" button on the phone.
Was going to try this but decided to port the number to a sip provider.