If you have a Google Play gift card, gift code, or a promotional code, you can use it to purchase a Truecaller Premium subscription.
To learn how to redeem and use your code, follow this official guide from Google:
https://support.google.com/googleplay/answer/3422659?hl=en
Once the code is redeemed and the balance is added to your Google Play account, simply:
Open the Truecaller app
Tap your profile icon
Select Upgrade to Premium
Choose a subscription plan
The gift card balance will automatically be used during payment
If the balance is insufficient, the remaining amount will be charged to your linked payment method.