Almost all numbers should be portable these days (prepaid/non-contract cell numbers are usually not, however), you should be able to take your number to any provider.
I went the other way, I got my phone number back in 2002 from Vonage, and I have since ported it to Broadvoice, and now ported it into a real Centurylink landline, VoIP telephony is nice, but there's some things it just can't do, like work in power outages, or support fax/modems/wired-alarm-systems worth a darn (some have finagled Vonage into working with them, but I was never so lucky). Same holds true of cable co's phone service, it's just bundled VoIP.