Internet connection sharing & its effect

I administer 2 LANs both of which have one Internet connection using 1 56k modem for each connection. We run Windows NT on one LAN and Windows 2000 on the other. At times we have 25 to 30 users using the Internet and everyone complains its to slow. A college suggest that it is due to our recent change of ISP. But on further investigation through my ISP I was told that our 56k bandwidth is split and shared between the number of users using the Internet at the time e.g. if we had 30 users accessing the Internet at the same time each user would receive 1.8k bandwidth. Is this true or is my ISP hiding something from me. To me it makes sense but my college disagrees.

Thanks
Nathan