Even though I'm not too familiar with ventrillo/teamspeak, I'm pretty sure that's it's going to be impossible finding a legit free server. Some other users here may have some knowledge as to where you can get a good free one, but if not it looks like you can get one pretty cheap here: http://www.maxfrag.net/
For 10 people it would be a cost of $3 a month, which really isn't that bad if you think about it.
Anyways I wish you the best of luck, and I hope the helps!
I use TeamSpeak 3 with my clan and it is great it is very light and it doesn't slow your internet connection. I think it isn't possible to have free servers
If you can't find a reasonably priced vent server you could go with skype conference calls. They are free and the quality is nice. You can also have 10 people in at a time.