First question: Your Apple Watch Series 6 has been around for several years, is this a new problem and you know it used to work OK, or has never ever made outgoing cellular calls?
You know the hardware in your Apple Watch is working OK because you can make outgoing calls and this uses all the same functions that would be used for incoming cellular. I suggest check the obvious even if it has already been checked, swipe up on the watch face to open Control Center and check you have not accidentally activated any of the special modes like Theatre, Do Not Disturb, Silent mode, or the Focus Modes which might block incoming calls.
Try selecting the Explorer watch face for testing, because the 4 green dots are a great indicator that you are really using standalone cellular.
My first suspicion is that your Apple Watch is not configured correctly as a second device on your iPhone's service plan. That is likely to be an error at your cellular network, but the problem should have been always visible since you setup the watch service plan. It is unlikely the network would meddle with your plan after it was working, simply because that would be extra work for the human.
The first place I would check is to make a cellular test call (outward) to any other phone you can see. (You might need to completely switch off your iPhone, or move to a distance where you are sure it can't make the call via the phone). Note the caller ID on the call from your watch. It should show the same number as your iPhone. If it has a different number it is certainly a problem in the network's activation of the plan for your watch.
