You can switch your current landline number into a google voice account to retain your phone number on a free Obi phone line.
Heres a landline to GV port demo;
Get ahold of an old Page Plus or Verizon cell phone that isn't being used. Port your existing landline to Page Plus for free through their website
Fill out the port form and put in your landline number and the old PP cell phone's ID number (from under the battery) and wait a couple of days for it to go through. You will know that the port is complete, when dialing your landline rings to the cell phone.
After the port completes you should have an active PP line with 2 dollars credit in the account DON'T WASTE IT!! You'll need it to accept a 1 minute call to verify the GV port!
Once the port is complete you can initiate the GV port. People have reported that it's wise to open a new Gmail account (ie not one that you have open a lot on your computer). You can then setup all emails that come to that account (all your voicemails...) to be forwarded to your main gmail account. Login to Google voice from that new account and port in the number. It will cost $20, and within 24 hours the port to GV will be complete.
Verify the number and you are all done!
for some good tutorials.