In CM have you configured 4599 as a device, route pattern or what? An immediate fast busy from another IP phone typically means CM doesn't know where to send the call or can't match the digit string with anything in it's DB. If it's simply an analog device like a cordless phone you could just create a route pattern on CM pointing to that gateway/route list.
You then need to build a pots dial peer on the router to point that destination pattern out your FXS port.
That's it in a small nutshell assuming that other devices and such are working properly.
Hope this helps.
