Voip over Wireless

Hi,
This is probaly my firs question, so please help. I have a sattelite internet in africa, i have 512down/384up link connection. In my main site voip works well, but I expand the connection wirelessly and have about 30customer sites. the internet works fine the customer site but voip(example skype) dont work there at all. it works fine in the main site. but in customer sites it righs and then it cuts the whole wireless connection? why? I dint configure any bandwidht for these sites. it just spreads the main bandwidh connections. in this case i think its Point to multipoint connections. so all the customer sites can also ping each other. how much probmlem this couses? Please give me any advice to fix this.

Thanks