Can't have more than 1 guest in the conference

I'm based in Russia. If this is the case. 
When I start a conference (usually with 1 guest) it works all right. But as soon as more than 1 guest wants to join, any video whatsoever stops for all of us. Presumably, only 2 guests can speak and the third is lost...video gets lost for all of us. Sound can remain sometimes, sometimes not. Very poor service.