Change advanced dial-up modem options on Mac
If your ISP gave you settings other than the phone number, account name, and password for your dial-up PPP account, use the panes in Network preferences to enter advanced modem settings.
Advanced settings include additional modem and PPP settings, as well as DNS, proxy, and WINS server settings.
On your Mac, choose Apple menu > System Preferences, then click Network.
Select your modem port (internal or external) in the list at the left, then click Advanced.
Click Modem and set the options that meet the requirements of your region and ISP.