Why can't the modem be used to dial-up?

The modem cannot function or be used to dial-up when connected to a Private Branch Exchanger (PBX).

There are 2 types of PBX - one for the analog line and the other for the digital line. The Private Branch Exchange (PBX) mentioned here refers to the digital line.
As the modem built-in the VAIO computer is a device designed for the analog line, connecting it to the digital line may cause a failure, heat or fire. In addition, some PBX's for the analog line does not meet the specification for the modem to work. To use the PBX line , please check the specifications of the PBX with the maunfacturer.

Note: The following information can be located in the online manual and user guide:
The internal fax modem can be connected to the subscriber telephone line. Do not connect it to the PBX (Private Branch Exchange). Connecting to the PBX may cause a failure or fire. Never connect it to the line for home telephones and business phones in particular.