Hi @786kk, welcome to our forum!
It sounds like you have installed a third party application that will display ads, or if you have another dialer application that contain ads. Are you using the pre-installed Dialer application to make calls, or have you downloaded another one?
If you are using the pre-installed one, try restarting your device in safe mode and see if these ads show up when downloaded applications are temporarily disabled.
Let me know how it went! 
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