I lined up a BlackBerry Curve, an iPhone, and a Treo 650 on my desk and put each of them through their paces. All three phones were able to initiate a call, and while in that call, add multiple other lines without issue. Each one lets you dive into your contact database or recent calls to find the number you'd like to add to the call, and I was able to add two lines to each call (that was all I had available at the time). The one bummer is that you have to add each call separately. There was no way to add the two other calls at the same time. But all three of these devices are very capable of making multiline calls with no problem. (I would suggest, however, that you don't activate speakerphone mode while doing this; I got a nasty screeching echo.)
Now for foonz. Foonz is different. To start a conference call, you have to text the word "start" to the short code 36669. It sends you back a text message giving you instructions on how to start the conference call. Basically, you text the word "start" again and add each number you want to include in the call. The numbers have to be cell phone numbers and you have to add them manually. No pulling them in from your contacts (though you can if you register with foonz). Once you've done that, you enter your name, and it sends your intended callees a text message letting them know they've been invited to a conference call. Once they dial the number in the text message, all the lines are combined into one call.
Compared with the regular smartphones' conference calling capabilities, foonz took about as many steps to get the call going and everyone joined in. The quality was good, and I didn't notice any of the ghostly echo with the call going through foonz's system.
The best part, of course, is that the service is free. Only your voice minutes are used, no extra charges apply.