How to stop Siri from asking what app should be used to make a phone call?

What Call? On an iPhone, you can make and receive various types of calls. Here are the different types:

1. These are standard voice calls made over the cellular network. They utilize your carrier's voice network to connect you with other phone numbers. Allow phone calls on your iPad, iPod touch, and Mac
2. FaceTime Audio allows you to make voice calls over Wi-Fi or cellular data using your internet connection. It provides higher audio quality compared to traditional cellular calls.
3. FaceTime Video enables you to have video calls with other FaceTime users. It uses the front or rear camera on your iPhone to transmit live video during the call. Use FaceTime with your iPhone or iPad - Apple Support (IN)
4. VoIP (Voice over Internet Protocol) calls use internet connectivity to make phone calls. Apps like Skype, WhatsApp, Viber, and Google Hangouts utilize this technology, allowing you to make voice and video calls over the Internet.
5. Some carriers support Wi-Fi calling, which allows you to make and receive calls over a Wi-Fi network instead of relying solely on cellular coverage. This can be useful in areas with weak cellular reception. Make a call with Wi-Fi Calling - Apple Support (IN)
6. The App Store offers numerous calling apps that utilize different technologies. These apps can provide additional features and options for making calls, such as international calling, call recording, and more.

It's important to note that the availability of certain call types may depend on your iPhone model, iOS version, and carrier.

To improve my understanding of the phenomenon, it would be highly beneficial if you could share a full screenshot (Do not crop it). Visual representations carry a greater impact than written descriptions.

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With the limited information that you have provided, It is hard to even guess the problems faced by you. 

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Please re-post with detail so that someone else can help.

Go to Settings > Apps > Default Apps > Calling and make sure the right app is selected.