I used to be a SysAd sa isang call center. Got hundreds of users. I forgot the dialer, pero I used this solution for their access:
http://www.endian.com/es/community/overview/#.UmWUdScues4It's a Linux based firewall solution. Ginamit namin is the free edition. May proxy, firewall, VPN, QoS and other functions yan. Fine-grained ang security, tied ang usernames sa ip/mac address. Pwede ka ring mag-allocate ng certain bandwidth per subnet.
Common problems? Makukulit na users na gustong mag-download/stream, he he he. Access to non-work related sites. As for the bandwidth, pwede kang mag-allocate ng certain bandwidth for VoIP. SEparated to sa http traffic.