Switchings From Comcast Voice

Currently I have a combination Comcast box that handles voice and internet. I received the Voipo equipment today and also purchased a new Motorola 6121 Doscis 3.0 cable modem. My question is:
1. Can I hook up the new cable modem to test the system or will this deactivate the comcast unit or should I wait until I port the number over to hook up the new modem.
2. I set up temporarily the Voipo unit on one phone and I got a temporary number. A friend who also uses Voipo called me on my house number(which was still connected to the Comcast box) and it rang on the phone connected to Voipo. Strange!

Thanks