John,
This part of Skype's faq page may answer your question:
Why is Skype better than Net2Phone, ICQ, AIM, MSN, etc?
Because it works! Most Voice-over-IP applications don’t work from behind firewalls and NAT (Network Address Translation) devices. Nearly all broadband users are behind a NAT or firewall and so they cannot use VoIP applications. Skype is not a typical VoIP application – it’s P2P Telephony! Because of its advanced design, Skype works behind nearly any firewall and NAT!
High Call Completion Rate
No other Internet telephony system has a call completion rate as high as that of Skype.
Superior Sound Quality
Current Voice over IP and chat applications just don’t measure up. Skype has created proprietary software and partnered with the best acoustic scientists in the business to deliver sound quality superior to even your fixed telephone line!
Extremely Easy to Use
Current Voice-over-IP applications are difficult to configure and it seems the provider assumes its users are experts in engineering. With Skype there is no difficult configuration of hardware or software. You shouldn’t have to work to use our software, software should work for you!
Totally Secure Communications
Your calls are encrypted “end-to-end” and are thus totally secure.