I'm looking for a freeware/open source VOIP application for Windows 9x/98x that preferably can use:

- the open source Speex codec
- any SIP (standard VOIP protocol) provider

It would be nice if it could call land-line phones, too.

Recently I had Skype 2.5.0.141 installed and working (voice, didn't use video) without noticeable problems on Windows 98 SE (even though it is not supported, and hasn't been since version 1.3, I think). I tried installing a beta of Skype 3.0, but alas it wouldn't install. Is anyone using Skype 3.0 on Windows 9x?

By the way, Skype doesn't use the Speex codec. It uses iLBC. From what I read, iLBC is considered to be inferior to Speex (especially on lower bandwidth connections). Has anyone tried to get Skype to use Speex instead of iLBC?

I've just installed Skype 3.0.0.205 version(latest) over 3.0.0.190. No problems, working very good under Win98SE (I haven't tried a video conference).

You could try Miranda IM (check my signature). There are a few voice plug-ins under development, check the addons page as well as the forums. All open-source.