said by Stewart:
Unfortunately, there is no way to get such redundancy for local numbers. If the CLEC's switch fails, you're out of luck. However, if just your PRI goes down, e.g. by an errant backhoe operator or a drunk driver hitting a telephone pole, the CLEC can route your calls via SIP, analog lines, or cellular.
First, CLEC switches almost never fail -- this is why CLEC switches use the five nines (about 5 minutes a year of downtime) as a standard. Second, CLECS can and do offer Emergency Call Forwarding (sometimes free of charge, depending on the scope of the emergency) and Temporary Call Routing. If a telephone pole goes down, or an errant backhoe digs up a telephone line, there's a good chance it's going to clip your cable or (especially) DSL line also (as they usually run in the same ditch or on the same pole).