I've seen and played around with lots of ATAs (or some prefer TA for short), which are used by broadband VoIP providers, such as Vonage, AT&T CallVantage, Lingo, Broadvoice, Packet8, etc.

With so many ATAs on the market, it got me thinking, "What would I like to see in the feature-set of the 'ultimate ATA'?"

Well, here's my feature-set for "My Ultimate ATA":
1) Support for the G.729a codec simultaneously on BOTH ports.
Many ATAs only support this compression codec on one port
2) Not locked down or password protected.
Many ITSPs, such as Vonage, lock/password-protect their devices. So if you want to switch VoIP service providers, you are left hanging owning a worthless brick!
3) PSTN Failover in the event of power failure or Internet outage
This is only important if you have a secondary PSTN port to plug into the ATA. Useful in DSL deployments which often keep the analog PSTN line.
4) Dial over the PSTN (if available) intead of VoIP if you dial 911
This negates the need for E911, but again, it assumes you haven't killed your land-line.
5) Web-browser administrative interface for accessing advanced ATA functions and features
6) Supports fax machines
Many ATAs have difficulty with fax machines
7) Handles touch-tone digits
Some ATAs have difficulty handing touch-tones due to compression
8) ATA works well with TiVo using VoIP call
Often times if you connect a TiVo to an ATA's analog port for updating the TV guide listings, it fails. There are many frustrated TiVo users out there that killed their landline and have tried to use a VoIP call to perform the updates. This has to do with the fax modem built into the TiVo and the compression that goes on inside the ATA which affects the fax signals.
9) Supports STUN and other NAT traversal methods.
10) Can setup call forwarding from the web interface to forward calls to a specified number (i.e. cell phone)
11) Can setup advanced call routing and call forwarding rules
For example, the ability to define schedules for when calls are forwarded to which number. Another example: You can specify that it calls multiple pre-specified numbers "round-robin style" until someone answers and enters in a PIN to accept the forwarded call.
12) SIP Support (obviously)
13) Supports registering onto multiple SIP registrars.
This way you can for example, register on Vonage's server (for PSTN outbound calling) and also register on FWD (SIP-to-SIP user calls). Then you can initiate calls to either the PSTN or SIP users in a SIP database.
14) QoS support - prioritize VoIP packets
15) Let the ATA sit in front of or behind the firewall. (Most, but not all support both)
16) Plug and Play configuration to any VoIP service provider using the Web inteface
I should be able to type 192.168.1.x, logon to the ATA, select an ITSP (Vonage, Packet8, CallVantage), enter a credit card, and it will confgure the settings for me, transmit my MAC address and credit info to the ITSP. Of course, this won't happen anytime soon. No ITSP is that "open" --- yet!

What Happens if You Connect Too Many Phones to an ATA

Well, I know I missed a few features I've wanted over the past couple of years, so feel free to post some comments with your suggested ATA features.