At the moment I'm using an iPhone primarily, though I also have a couple of other handsets floating around the house and office (thanks for nothing Three!)
We recently upgraded the connection into the office, so we're switching over to an Asterisk powered PBX on Monday. I don't know much about Asterisk, VOIP, SIP or any of that stuff, but what I do know is that I can login to the PBX from any location using a SIP client.
On the Mac there are a number of softphone solutions which offer a range of features. What really impressed me was that they can do things like poll my existing contacts on the Mac seamlessly (OK - I am easily impressed at times!).
But what about the iPhone?
Most of the software I've come across is web based services with SIP features.
While they may work they're not exactly ideal. As I've no interest in using the other services that they offer logging into a web service before making a SIP call seems a bit dumb, as I have both 3G and WIFI on the iPhone.
In the tests I was trying last night with Nimbuzz using the WIFI in the house to get a connection worked reasonably well, but failed miserably when I tried to use 3G only (ie. disabling the WIFI connection). Nimbuzz also failed to call most numbers in my phonebook if I included the full STD codes - even though it automatically fills in the "+" when you dial manually.
So can anyone recommend or suggest a SIP client for the iPhone that only does SIP?