In R4.5, Be includes detailed documentation on building custom initialization strings. You can access this documentation by clicking the „AT Commands…“ button in the Modem panel in the DialUp-Networking Preferences. Below are the pre-R4.5 instructions.
When you set up a modem under MS Windows, settings such as modem speaker volume and enable/disable call waiting are taken care of for you. Under BeOS, you need to understand the modem initialization string to control these features. The best place to look for a complete table of init string options is always in your modem’s manual or on the vendor’s web site. Here are some common ones that apply to US Robostics modems.
A new entry in /boot/beos/etc/modems.ppp might look like:

3COM_V90_mystring AT&F1S0=2L0S32=34

Explanations:
AT&F1 means: load default modem hardware configuration.
S0=2 means: modem answer incoming call after 2 rings.
L0 means: modem speaker set at low volume. To turn off speaker, use M0.
S32=34 means: diable X2 protocol, enable V.90 protocol.
Note: For every USR modem, the string AT&F1 should work fine, and you can append some or all of these options as you seee fit.
For more information, please go to http://consumer.3com.com/support/, select his/her product, and select „manual“ button. Download the .PDF technical reference file for full information about the initilization string.