While a modem can enter PPP mode from command mode at any time, the intent of PPP is to connect to the network. Until there is a network connection established and the modem has registered, it is better to delay entering PPP mode as you may report errors that will only persist until the modem registers. It is generally recommended that the host check the status of the modem before proceeding with a PPP connection. The Status can be checked using the "ATS57?" command.
An alternate method of entering PPP mode is to send the word CLIENT to the modem. The modem will respond with SERVER. This is NOT an AT command. It is neither preceded by an "AT" nor succeeded by a CR. Exiting PPP mode is performed using the DTR control signal.