Hi rumeysa244,
We appreciate you reaching out to us.
We’d like to gather additional details that may help us be in a better position to assist you.
Is the issue only happening with certain contacts?
Also, do you have a different Apple device that is signed in to the same Apple ID? If yes, are you experiencing a similar issue when initiating a call on your other device?
In the meantime, let’s check for a software update and make sure that you are running the latest version of iOS.
While it isn't inherently assured that an update will resolve the issue, updating to the latest software version for your device can help isolate potential software issues.
Kindly secure a backup and update.
How to back up your iPhone, iPad, and iPod touch
Update your iPhone, iPad, or iPod touch
Take care!