You can change the "Call Audio Routing" setting of iPhone's settings to be enable to receive call by the earphones automatically. Please follow the operation below.
Open the iPhone's Settings ⇒ [Accessibility] ⇒ [Touch] ⇒ [Call Audio Routing]
The default setting of the "Call Audio Routing" is "Auto".
[✓] Auto
[ ] Bluetooth Headset
[ ] Speaker
Please check "Bluetooth Headset" as below.
[ ] Auto
[✓] Bluetooth Headset
[ ] Speaker