"Fring" is a Instant Messaging and VoIP client for the iPod and iPhone Touch. It's a free download, and I decided to try it.
When you start Fring, you have to register for a Fring User ID -- before you even enter your IM, SIP, or Skype contact info. That suggested to me that it was doing this stuff through a central server.
I setup my MSN, AOL, and Yahoo IM contacts and sent a few IMs. It sets your IM status to "Mobile with Fring".
Then I setup my Skype account. I bought some SkypeOut credit. But I was never able to get it to place a call outbound. I deleted my Skype login on Fring, then tried to set it up again -- but it wouldn't let me login.
I also configured a SIP account with a VoIP carrier, Vwave.net, an NGTelecom Company. NGTelecom is a sister company; they're a very modern SIP VoIP provider: they have an Acme Packet SBC and DNS SRV records. I entered my SIP username, SIP authentication password, and the Vwave domain, "vwave.net".
The Fring system did a proper DNS SRV lookup, then registered through the NGTelecom Acme Packet SBC. I noticed that it registered from 220.127.116.11, an IP address assigned to CWIE, LLC of Tempe, AZ.
The CWIE IP Address did not stay registered after I left the Fring application on my iPod Touch. In fact, when I restarted Fring, it wasn't always able to re-register; the progress spinner would keep on spinning.
Call Quality Problems
While Fring was registered, I placed several (maybe 6) calls. Two (2) of them carried intelligible speech. In a couple of cases, I heard weird feedback, on my iPod Touch, before the call was even connected. But I don't know of any similar examples from The Cisco "Duck Quack" page of Voice Quality Symptoms. In some of the call cases, I had one way audio. (Vwave's Acme Packet SBC generally solves all NAT problems for moderately-well-behaved SIP clients, but symmetric RTP is required if the RTP flows through a NAT device or stateful firewall.)
Comments and Conclusions
I was hoping for a SIP client that actually runs on the iPod Touch. Fring apparently sends the call across the Internet, so that means your SIP proxy has to be reachable from the Internet.