Also, I just remembered that I think if you plug a phone into your phone jack, even if you don't have landline service, you can still use it to call 911.
I'd try it, but, uh, that would mean calling 911. But I do think there's a very real chance that that's the case, so look into that first.
Also, a dead phone could happen to a landline, too. I guess having backups is nice, but it seems unnecessary to me.