I see two issues that plague FLOSS VoIP apps and do not allow their widest adoption. The first VoIP app that gets these, should get fame and glory.
First, the majority of users are broadband users, with a router that protects the inside systems. It is not possible for an outsider to initiate a network connection to a system inside the LAN. If both users that try to communicate have this typical network configuration, then the current tools say something like «You have a symmetric NAT, and at the moment the way to fix this is to either put your computer in the DMZ or enable manually port-forwarding for specific ports.» Ekiga discusses this issue at Ekiga behind a NAT router and directs the affected users (when using the program) to that page. Twinkle (QT-based SIP-phone) shows you a dialog box with the exact ports to enable for port-forwarding on your router.
The issue of a symmetric NAT can be solved in most of the cases by using the UPnP protocol. If the router supports UPnP (most do by default), then the VoIP app can enable the port-forwarding by itself, transparently from the user, and it will just work.
Until recently there was no good UPnP library, which was probably the reason for the lack of support. However, this changed with GUPnP.
The bug report to add (G)UPnP support to Ekiga is Bug 337166 – UPnP for firewall/NAT penetration.
The other issue is the difficulty in configuring your third-party VoIP SIP account, that allows you to make inexpensive voice conversations to telephones and mobiles. The reason why it is difficult is because the user has to figure out the SIP server and other configuration details. The terminology is confusing, registrar, domain, etc.
Ekiga has good support in configuring Diamondcard. The user is presented with a Wizard when configuring for the first time Ekiga.
What needs to be done is to put together the details for each provider and maintain a list at ekiga.com. The client would have a copy of the list, and allow on-demand updates. The bug report, Bug 547215 – Ekiga should know all SIP/VoIP providers, allow easy account addition.
Update 11Aug08: Wiki page with PC-to-phone providers and their details.