Thats a workaround for a second PSTN GW or Second trunk
Our recommendation is alternate lines, FXOs or SIP trunks to provider via SBC or SIP Provider directly.
All above using Route Groups and Route Lists
I know SIP Provider includes an extra cost, but we definetly need Outside world access in an event of failure 911 emergency which routing back to an IP Phone does not provide.
Let u sknow your thoughts
Thanks