I did not see any incoming calls and just got missed, why?

Do you mean:

• I got a Missed Call notification but my phone didn't ring!
  

This is not so common but possible when you use Silence Unknown Caller or Call Blocking & Identification i.e. filter options.

To learn about the same, please follow the Apple website.

Also, 

Please ensure Do Not Disturb feature is turned off:

1. Go to device Settings app
   
2. Tap Do Not Disturb 
   
3. Ensure it is disabled (or turn it Off)
   

Please ensure the device time & date is set to Automatic:

1. Go to device Settings app
   
2. Tap General
   
3. Tap Date & Time
   
4. Ensure Set Automatically is enabled (or turn it On)