In BeOS R4 and prior, adding a custom initialization string to BeOS entailed editing /boot/beos/etc/modems.ppp. Since this is technically a system-level file, doing so was technically a bug in the operating system, since users should never have to touch system-level files. In R4.5, this situation is rectified with the addition of a new dialog to the DUN panel.
If your modem doesn’t appear in the DUN modems picklist, click the Custom button and give your modem a name (the manufacturer and model would be a good choice). From here, you have two ways to proceed. If you already know your modem’s initialization string (e.g. you’ve got the modem’s documentation or you’ve found the manufacturer’s web site), you can simply type or paste it into the dialog and click OK. You may want to search through the massive database of init strings at http://www.modemhelp.org/.

If you can’t find the init string for you modem, you can try and build a string from scratch. Some of the most basic modem commands are accessible from the new AT commands picklist, while advanced documentation is now provided by Be and can be accessed by clicking the AT Commands… button.

Note: New modems are not added to the modems.ppp file, but are stored in configuration files living in the ~/config/settings/DUN hierarchy.

If you already have a working modem but want to add commands to the existing init string, the same dialog will let you make changes to your existing modem.
The instructions below are for users of BeOS R4 and earlier.

Many (in fact, most) modems not listed by default in the Dial-Up Networking panel can be made to work in BeOS by adding their initialization strings to /boot/beos/etc/modems.ppp. Adding a section or new modem to this file will cause it to appear in the DUN panel. (See also Ultimate init string database).
However, this file is write-protected, so you’ll need to give the file write permissions before you can edit it, then turn them off again when you’re finished. You can do this with the SetPerms Tracker add-on, or via the shell’s chmod command. Alternatively, see the tip Simplified read/write permissions.

1. Open a Terminal window and cd to /boot/beos/etc.
2. Type chmod a+w modems.ppp
3. Open modems.ppp in your favorite text editor.
4. Add a line at the top that reads similar to „MyModem ATZ“ with your init string in place of ATZ. If there are any spaces in the modem’s name, replace them with underscores.
5. Save and close the file.

You will need to find a working init string for your modem to make the change useful. The easiest way to find a working init string is to steal it from another operating system on the same computer. Alternatively, read the modem’s manual or check the vendor’s Web site.