You can set Truecaller as your default dialer in two ways:
Method 1: Through the Truecaller App
Open Truecaller
Go to Settings > Calls
Under Troubleshoot Calling, tap Set as Truecaller default
Method 2: Through Phone Settings
Open your phone’s Settings
Tap on Apps & notifications
Select Default apps
Tap Calling app
Choose Truecaller
When prompted with "Use Truecaller instead of Phone as your calling app?", tap Yes