Error 676 in dial up connection

I am using Lucent modem for my dial up connection and dialing two number one is my company’s VPN number and another is normal MTNL dial up connection number. it is connecting with the VPN number but when i am dialing the MTNL number on the same line after disconnecting it is giving error 676 (that phone line is busy). and on the different machine it is connecting at the same time. Could you help me out from this?