Beginner: Yun Cellular WAN VoIP Solution

Hello all - I just stumbled across the arduino yun and arduino products recently and apologize in advance for any lack of knowledge.

However, I'm looking to obtain a WAN cellular connection via the Yun and pass the WAN to the LAN Eth connection on the Yun to a VoIP ATA.

I ordered the Yun and haven't received the device yet for testing purposes and was hoping the community could shed some light on the situation before hand. I've done some digging and am trying to better understand the Yun in terms of its routing capabilities as well as the Ethernet Shield and if the Ethernet shield is necessary for this project?

I know ATT provides a similar product via the Wireless Home Phone & Internet. However, I simply want to utilize cellular data for WAN connectivity to pass to a VoIP ATA that I can plug an old trimline phone in and place calls.

If the Yun or Yun-Ethernet Shield combo are not the right solution, please let me know of any other options available.

I'll be sure to update/delete the post if I stumble across any more information or answers.

Thanks again!