Well, to dish all of what I use for calling, here's the list:
Which regularly uses Skype and Google Voice (GV as a Click2Call Service)
Skype for Mac & PC
Used in combination with Skype (via SkypeIn) and Gmail
Home Phone/Landline (because almost everyone needs one… I think.)
Of course, since my iPhone 4 matters to me the most and is vital to my communications, I use the combinations of Skype and Google Voice together with my iPhone 4 to harness the full function of both services to bringing communication to my device.
In an aspect of usefulness, it does save me a lot to call using Skype/Google Voice combined with my iPhone. Sometimes, one can overrule another and save me more! For instance, my carrier, AT&T, would charge $3.51/min. to call a mobile in the Philippines from my home country, the U.S. Skype doesn't include that country in the list of countries that is included in its world unlimited subscription and a regular connection would be 17.4¢/min. Google Voice overrules them all by making all calls to the Philippines 15¢/min., which puts me in the comfort zone of calling with Google Voice over Skype.
While it sounds confusing, it really saves a whole load of money. In addition, a lot of countries are supported on my subscribed unlimited world plan on Skype (of which all I really care about is U.S., Canada, and Hong Kong and incoming calls.) Being able to use it in combination with my iPhone 4 and Google Voice just makes it even better!
Finally, I'm willing to try out the Vonage Time to Call™ app to examine its call quality, value, ease-of-use, and usefulness. I'm always open to trying out new things and I've always wanted to see what Vonage can do!
(Fun/Random Fact: I pass by Vonage's Headquarters in Holmdel, NJ as I go to classes in my college <5 minutes away! Haha)
Most of my calling is done at work - a cheapy Panasonic cordless phone on a Linksys PAP2T ATA adapter, with VoIP through VentureVoIP. Have been entirely VoIP for 18 months here.
When out of the office I use my iPhone. Haven't really tried any extra apps - I make few enough calls that the cost isn't a concern.
At home I also have a cheap Panasonic cordless on a PAP2T - all VoIP for well over three years now, initially on Xnet then some company I can't remember that charge my credit card $10 a month. Only thing it's used for is so my mother-in-law can call.