Always make sure that you're using an analog telephone line before you connect it to your Apple computer's internal modem.
If you plug a PBX or digital phone line into your computer's built-in modem, the line may cause damage to your modem. The Apple Internal Modem is designed for use with analog phone lines—the type that's commonly found in residential homes. PBX and digital phone lines are typically found in hotels and businesses, and require dialing "9" or another number in order to obtain an outside line. These phone lines carry a higher voltage than residential analog phone lines, which can cause damage to your computer's internal modem.