Hi Peter,
We use Asterisk PBX (an old trixbox box). We use telnet to talk to the "manager" service of the box and can do couple interesting things. Our actual implementation is not in WinDev but since it's telnet, it can be done quite easily.
[
www.voip-info.org]
For any other PBX I don't know. But I don't believe much in WinDev's phone functions for that: they all require a modem and work with TAP lines directly. So there is no VoIP available (as far as I know). There was a VoIP demo once, but they probably have done it by hand.
Best regards.