The 3Com 510 is setup to handle a modem for internet access but won't talk to a Sportster modem properly.
If you use windows to control a Sportster 56k, you can access a dialup service every time.
If you use the 3Com 510 to control the Sportster 56k, it will only work after being configured by Windows. (i.e. - turn off the modem and turn it back on, the modem control string is not retained and the sportster can't access the dialup service).
Can you configure the 3Com 510 modem control port to pass the proper setup strings for the modem?