As a VoIP user I would first see the customer service of any service provider, because in the beginning when I started using VoIP for the first time I encountered with some service provider who do not have good customer support rather I should say NO customer support. In the beginning I use to select the services which use to offer the less calling rates but after experience I can say that less calling rates plus a good customer support is necessary if a service provider wants to retain its customers. These days I am using http://www.vopium.com which is a new mobile VoIP and it has both good customer support and also cheaper calling rates.