Good question. The answer is:
If you simply wanted to use multiple OBiTALK devices, registered to the same Gmail account and its Google Voice number, then yes, you can do that. The process authorizes the devices to access the Google Voice service on the account, and you can authorize as many devices as you wish.
However, you can't do this for a security panel. The panel needs a dedicated phone number, otherwise it won't work properly.
Aside from that, it's an unreliable and unsupported solution. The security alarm monitoring industry has moved to using either a 4g LTE wireless modem, and/or a direct IP connection via a gateway both supplied by the monitoring company. You are much better off upgrading your system with current technology.