Using Voice over IP operators became fashionable mainly because of the low tariffs they offer in comparison with the traditional operators (who are sometimes charging as if they had "old style switched networks" but who are using voip themselves anyway). Several alternative providers had been suggested to me but in the end it seemed they all were from the same company. The options I was considering were different brands of Betamax:
"Betamax GmbH & Co KG is a company registered in Cologne, Germany. In December 2005 it took over all of the VoIP services previously run by Finarea SA, and has since added a few more. With one exception, all of these services offer the same features; the only difference seems to be in the rates charged to various destinations at any one time, and in the color scheme used on the web pages and soft phones.A more complete price comparison of the voip offers from this mysterious company can be found at http://backsla.sh/betamax.
There are some indications that Betamax might just be a front for Finarea which still operates traditional PSTN call-by-call services in a number of European and Asian countries." (full post)
Note: It appears the prices change and thus keeping an eye on them might be advisable (see more here) . My experience is that prices might not be exactly as advertised. For example, 12voip charges differently if you use your voip equipment or if you use their softphone, although only the latter is published.
More general comparisons of VOIP prices, including Skype, etc, can be found here.