I'm looking for a, preferably free, multi-SIP client for Windows that does support g729 codec.
EDIT: Per the comment below, g729 is now free. Definitely will open up more options for testing and VOIP solutions.
According to en.wikipedia.org/wiki/G.729#Licensing : "As of January 1, 2017 the patent terms of most Licensed Patents under the G.729 Consortium have expired, the remaining unexpired patents are usable on a royalty-free basis. Thus, G.729 can be used free-of-charge."
Take a look at Bria. I use it at work and it is great. It supports g729.
I also have tried Ekiga and it does not support g729, likely because the g729 license is not free.