I have an Apple Watch with number sync enabled.

Well you posted in prepaid. Number sync does not work with prepaid service

Is it true that with numbersync on, if my cell phone is off then I can’t make or receive call from the Apple Watch ?

Not true.  

If you have post paid service, with both your phone and the Apple watch set up with number sync, a cellular Apple watch will work when the phone is off, or not in the same location.

Hello @Klsauzier, let's get you connected!

To answer your question, no, that is not true. With NumberSync, you can use your Apple Watch to make and receive calls even when your iPhone is off or out of range. NumberSync lets you use the same phone number on both devices, so you don’t need a separate plan or SIM card for your watch. 

We recommend desynchronizing your devices and synchronizing them once again and see if that fixes the issue. You can follow the steps below. 

Unsync your watch from your current iPhone

1. Select the Watch app.
2. Once you see the My watch screen, press the picture of the watch to remove.
3. Select the information icon.
4. Select Unpair Apple Watch.
5. Select Keep Plan.

Sync your watch from your current iPhone

1. Go to the Watch app on your iPhone and follow the prompts to start pairing.
2. Select Set up Cellular. Syncing to another iPhone with a different mobile number? Select OK when you see a message asking if you want to sync with another phone. (If you upgraded to a new iPhone you won’t see this message.)
3. Enter the Emergency 911 address. This is the address where you’ll use the watch most often. 
4. Make sure to check the box and read the Important Information for NumberSync on the 911 screen. 
5. Select Ok on the Sync requested screen when you see a message that NumberSync should be available shortly. 
6. Follow the prompts to finish setting up your watch.

You can find this information on our website under NumberSync. 

Please get back to us and let us know if this works for you. If not, we can look at other options for this case. 

Thank you for visiting AT&T Community Forums.

Andrew, AT&T Community Specialist.

Tried steps above and still have an issue dialing out or receiving calls when phone is off or in airplane mode. Apple Watch has a paid line 

Hi, thank you for your reply.

Please meet us in a DM for better assistance. We have already initiated a DM for you. Waiting to hear from you.

Donald, AT&T Community Specialist

For some reason can’t  access DM. 2 factor authentication can’t be sent. My cell number that’s on the account is not in the list. I don’t even see my wife’s 

Seems the issue is with the watch line. Somewhere along the way between the iOS upgrades, the watch when disconnected to the phone and WiFi does not detect the watch line. The LTE icon in control center on watch does not turn green as it used to to indicate its own line and access to make and receive calls independently of the phone. 

@Klsauzier 

Check your online account to make sure you have an active watch line.

 Checked and definitely have an active wearable line.  

like I said before, in control center on the watch when disconnected from the iPhone and WiFi, the LTE icon used to turn green along with signal indicator. That’s is no longer happening.