FWIW, I picked up my A5IT a few days ago. I'm really enjoying it. I've played with several SIP clients in the past couple of days and am really liking 3cxphone. It is the replacement for Sipagent. The interface is pretty nice except for the logo, but it's free. I don't know how it compares to Sipagent but besides that it works* it has an audible ringer. Seems it rings only one time, but this should be sufficient. It's a lot better than ringing no times.
To be clear, it rings once and maintains the incoming call until you answer or it goes to voicemail. Not to be confused with giving you only one ring and no time to answer the call. I'm using it in conjunction with Google Voice. On one A5IT, Sipgate provides the backbone. On the other, we have Ipkall with Iptel -- a less favorable solution for me but Sipgate registration is bonkers since Lifehacker featured an article on using their service w/GV.
*I haven't tested call quality b/c I didn't have anyone to call this time of night and my wife wasn't available to grab the other line.
I can verify that 3CXPhone works as a nice VoIP client on the A5IT, as does Fring. I am juggling both at the moment and each has a flaw. Fring doesn't have a ringtone on incoming calls so that's annoying. 3CXPhone rings once as you mentioned, but on outgoing calls it shows my sipgate number and not my Google Voice number. Fring gets this right. It seems this should be an issue with GV Dialer, but I'm using GV Dialer with both VoIP clients so it must be client-specific.
Each has a gap and the search for the perfect VoIP phone for the A5IT continues...
If anyone has a nice solution to either of these problems please post it!