Of course they'll need the Modem pop-up menu set to whatever their modem actually is.
Most Performa models with a built-in modem have either a Global Village or GeoPort modem.
You've got 2 standard options on iMacs:
iMac Internal 56k and iMac Internal 56k (v.34 Only)
...OR Apple Internal 56k and Apple Internal 56k (v.34 Only), on more recent systems.
The Connect via pop-up menu has different choices, depending on what their hardware is. Generally, for any internal modem, set it to Internal Modem, and for any external modem, set it to Modem Port.