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.
- (See video)
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.