Why would you care what OUTGOING line phone number you have?
If you have to subscribe for a few $ a month, but save $100/month, wouldn't you be better off with VOIP in addition to your existing service?
I can understand if you don't make enough calls to warrant it.
That said, I'm VOIP all the way.
Cordless Phone, ATA adapter & ADSL Modem are on a UPS .. which should keep them going for a few hours during a blackout/other.