One of the big differences between traditional carriers and VOIP providers is the flagfall. Pretty much all VOIP providers don't charge a flagfall and this makes a big difference.
For instance a traditional carrier offering (sayme Homeline complete) will cost you 35c flagfall plus one billing increment only to get someones voicemail
Using a voip provider that provides per/second billing would cost you about 5 cents for the same 10 second call.
In regard to landlines calls the VOIP offerings are REALLY competitive. There are providers like freecall that offer 1c/min nationally and others that offer 10c untimes nationally. (I've read some statistics somewhere where the average call length is around 3.5 mins)
If you have a smart enough ATA you can combine the best bits of all plans.
For instance.
In my situation I use my ISPs offering iinetphone for 10c untimed local and mobile calls at 29.9c /min biller per second.
Then for STD calls I use astratel as they offer 10c untimed nationally.
For international i also use astratel as there rates are really cheap and they also offer 19c untimed to 22 countries including the UK, singapore, USA...
I also set up my father in laws laptop in singapore so he uses a softphone (same as skype as far as he's concerned) but when he 'dials' us. its our home phone that rings, not our computer.
Bare in mind also that everyone on the VOIP bandwagon can also pretty much call each other for NOTHING.
In regard to the cost of the ATA.
Mine payed for itself within 3 months by taking our ~130 bill down to about $60.
Food for though