If the AirPods are in your pocket and in the case then the calls should not be transferred to them.
I recommend that you remove the AirPods from the Bluetooth list of devices by opening Settings ➔ Bluetooth ➔ Locate the AirPods in the list and tap on the "i" to the right: Then select Forget this Device.
After that Reset the AirPods as shown below and then pair them again to the iPhone-
Reset your AirPods
Put your AirPods in their case
Close the lid. Wait 30 seconds, then open the lid.
On your iPhone, iPad, or iPod touch, go to Settings
Go to Settings > Bluetooth and tap the "i" icon next to your AirPods.2 Then tap Forget This Device, and tap again to confirm.
Reset your AirPods
With the lid open, press and hold the setup button on the back of the case for about 15 seconds, until you see the status light flashing amber.
Reconnect your AirPods
With the lid open, place your AirPods close to your device. Follow the steps on your device's screen.
Axel F.