RAS dropping down 1of the 2 connections

I have an NT4.0 server that has SrvPAck 6.0 on it. It has two external US Robotics modem. What is happening is that when someone is connected on port 1 and another one connects, he can’t make it through. Then when the 1st one is disconnected the other can connect easily on any available port.

I haven’t done anything actually, I am thinking of re-applying SrvPAck 6 again. But I thought that some or one of you might have faced this idea so he could be of help to me.

Thanks in advance.