Some things on my checklist:
Cneck all preferences and remove any redundant downrev ones leftover from upgrading to 8.6. Check that background printing is off and disable print monitor/spooler extensions. Disable QuickCam extension if present. Restart.
Appletalk inactive in chooser, and Appletalk control panel set to Remote Only(IR should have worked OK also), configured not to make active upon closing CP. System needs to come up with Appletalk off, and not require that it be turned off each time.
TCP/IP set to PPP (make active when needed if you have this option)
Power cycling the modem then retrying to see if it is left busy from a previous attempt.
Verify proper hardware handshake modem cable (cable below is for US Robotics Sportster, yours may be different but Mac miniDIN pins 1 and 2 must be received and driven properly otherwise port will be unresponsive)
DB25<-->miniDIN8
2<--3 (serial data to modem)
3-->5 (serial data from modem)
4,20<--1 (Mac pin 1 drives DTR and RTS)
5-->2 (Modem CTS drives Mac pin 2)
7<-->4,8 (ground)
All other pins both ends are no connects, except for ground shell <--->ground shell
If you have a terminal emulator program like Microphone or Clarisworks in communications mode you can attempt to talk to the modem directly from several different Macs starting with a working one just to get the syntax down. It varies by modem, and a modem manual helps but there is usually something like at&v or ati that returns modem info. If you can get any info back the port is open and script issues may be at the heart of things. But if you cannot even communicate direct to the modem the script probably is not getting there either, and I would suspect either that 8.6 sees a busy hardware handshake signal or something is seizing the shared modem/printer port and not letting go. Some terminal emulators allow you to specify hardware vs software handshaking and if only software handshaking mode gets thru the hardware handshake signals will need more investigation.
Cneck all preferences and remove any redundant downrev ones leftover from upgrading to 8.6. Check that background printing is off and disable print monitor/spooler extensions. Disable QuickCam extension if present. Restart.
Appletalk inactive in chooser, and Appletalk control panel set to Remote Only(IR should have worked OK also), configured not to make active upon closing CP. System needs to come up with Appletalk off, and not require that it be turned off each time.
TCP/IP set to PPP (make active when needed if you have this option)
Power cycling the modem then retrying to see if it is left busy from a previous attempt.
Verify proper hardware handshake modem cable (cable below is for US Robotics Sportster, yours may be different but Mac miniDIN pins 1 and 2 must be received and driven properly otherwise port will be unresponsive)
DB25<-->miniDIN8
2<--3 (serial data to modem)
3-->5 (serial data from modem)
4,20<--1 (Mac pin 1 drives DTR and RTS)
5-->2 (Modem CTS drives Mac pin 2)
7<-->4,8 (ground)
All other pins both ends are no connects, except for ground shell <--->ground shell
If you have a terminal emulator program like Microphone or Clarisworks in communications mode you can attempt to talk to the modem directly from several different Macs starting with a working one just to get the syntax down. It varies by modem, and a modem manual helps but there is usually something like at&v or ati that returns modem info. If you can get any info back the port is open and script issues may be at the heart of things. But if you cannot even communicate direct to the modem the script probably is not getting there either, and I would suspect either that 8.6 sees a busy hardware handshake signal or something is seizing the shared modem/printer port and not letting go. Some terminal emulators allow you to specify hardware vs software handshaking and if only software handshaking mode gets thru the hardware handshake signals will need more investigation.