If you've been looking around for a solid VoIP/SIP client for the iPad or iPhone, consider CounterPath's Bria.
For all the excitement around VoIP on the iPhone and iPod touch (including high profile apps like Skype, Vonage TalkFree for Facebook and Line2), relatively few developers have gone after the corporate side of the voice mix. With so many large enterprises using Asterisk-based PBX replacements for VoIP telephony, a solid SIP client for iPhone and iPad seems like a natural winner.
Bringing the advantage of its long pedigree in desktop VoIP clients, CounterPath now delivers some of that savvy to iOS with Bria. On the Mac, Bria is the successor to the capable EyeBeam and free X-lite softphones; now shipping on both the iPhone (US$7.99) and launched this week for the iPad ($14.99), it provides some of the key features needed in a mobile voice solution. Bria works out of the box with major IP-telephony service providers (ITSPs), but keep in mind it does not include service with the app; you need to be using a hosted service or your company's IP telephony infrastructure for Bria to work.
The three most important considerations for any softphone app are call quality; call quality; and, of course, call quality. I was able to make and receive calls on several WiFi networks using Bria and a Fonality PBXtra server (built off the Trixbox Asterisk platform). While I heard a bit of near-side echo and low-bandwidth choppiness during some calls, most of the time the audio was as good or better than it would be on a conventional iPhone call, and less likely to drop. The people I called reported my voice quality as 'awesome' and 'really spectacular,' so it seems the artifacts I heard from time to time were on my side only.
It's easy to set up Bria; the app takes the same credentials and server info you use for your desktop VoIP/SIP softphone. Once you enter your server name and authorization, you can begin making and receiving calls. Bria is optimized for WiFi use, but it will work over 3G if you enable that option in preferences; however, you're at risk of chewing up your wireless data allocation if you make a lot of calls that way, and quality will suffer a bit. On the flip side, you are not using any voice minutes, and for calls to colleagues on your internal phone system you will appear to be calling from your office extension -- a great trick when you're actually enjoying a piña colada by the pool.