On a LINE port to PHONE port bridge, the OBi adds only a few milliseconds of delay, not enough to be noticeable.
However, why would you do that? Skype (with appropriate plan) and NetTalk are cheap services that appeal to heavy users seeking very low cost. IMO, that makes no sense for your situation; you already have Google Voice, which is free. Your secondary service would only be used in case of problems; minute usage should be very low. So, just pick a high quality provider with good support, such as Callcentric or Anveo. You won't need any special hardware, as the OBi supports them directly.
An ATA with two Ethernet ports can work three ways:
1. Between modem and router, for QoS.
2. As a stand-alone router, for a user with only one computer.
3. As a two-port switch. A computer or other device can be connected behind it, so it doesn't "consume" a port on the router.