Set up a dial-up modem to connect automatically on Mac
Set up your modem to connect to the internet automatically when you use an app that requires a connection, such as Safari or Mail.
After your modem is set up, whenever you open an app that requires a connection to the internet, the modem dials your ISP and connects.
On your Mac, choose Apple menu > System Preferences, then click Network.
Select your modem in the list on the left.
If your modem isn’t in the list, make sure it’s either installed in your computer or connected to the USB port if it’s an external modem. Click the Add button at the bottom of the list, click the Interface pop-up menu, then choose Modem. Give your modem service a name, then click Create. Enter the settings for your PPP account.
Click Advanced, then click PPP.
Select “Connect automatically when needed”.
Select other connection settings, such as “Disconnect when user logs out”, or “Disconnect when switching user accounts”.