Most organisations find the free services unusable.
There are companies that offer “free” conference call services, but of course when something looks too good to be true, it normally is.
A new customer costs a conferencing company $50-$100 (in marketing costs) before they even use the service, so companies that offer seamingly free services can’t afford many free users.
These services have limitations that force most of their customers to use the paid version.
However, this “premium” version is still using the same network and systems that are cheap enough to give away free.
Summing this up: these services are not free and there are reliability questions.
How much will a failed meeting cost in the attendees time, and your credibility?