iPhone not vibrating for incoming call while on a call

If your iPhone isn't vibrating for incoming calls while you're already on a call, it's likely because your phone is automatically set to silence incoming calls during an active call, meaning it won't vibrate either; this is a standard feature on most phones to prevent interruptions during a conversation. To check, go to your phone settings, navigate to "Phone" and ensure the "Silence Incoming Calls" option is turned off if you want your phone to vibrate for incoming calls even while on a call. 

Other things to check:

• Do Not Disturb: Make sure Do Not Disturb mode is off on your phone. 
• Vibration settings: Go to "Settings" > "Sounds & Haptics" and verify that the vibration option for incoming calls is turned on. 
• Physical switch: Check if your phone's physical silent switch is accidentally engaged. 
• Restart your phone: Sometimes a simple restart can resolve temporary glitches.