There are a number of leased line backup options to choose from.
The most popular choice for a backup connection. If your leased line goes down, you'll still be able to access the Internet. Your upstream speed will probably be reduced, as will the quality of your connection.
It's not ideal, but frankly it's cheap. And hopefully you won't have to put up with it for long, as most leased line faults are fixed within hours, not days.
Another Leased Line from a different carrier