In order to set Truecaller as your default dialer you need to enable "Missed call notifications" by doing the following:
- Open Truecaller and tap on the Menu .
- Settings .
- General > Scroll down to 'Missed call notification' and enable it. Once that is done you'll be asked to "Allow notification access", enable it and you're all set.
You can also allow Notification access within your Phone settings > Apps > Truecaller > Notifications.
Note that Truecaller does not replace the call screen which appears after you initiate a call where you can enable the Speaker or create a Conference call.