Let me try to explain. For at least the AT&T version when someone calls the number for the phone it will ring on the phone and the watch when each is connected via bluetooth, showing the incoming call on both devices. When the watch is connected via numbersync and both devices are in bluetooth range, both devices also show the incoming call. As soon as the devices are out of bluetooth range and disconnection from each other happens, the purpose of numbersync is to use the e-sim in the watch to "capture" that incoming call to the phone and send it in "real time" to the watch. If numbersync is turned off or not activated then when the phone is called then auto forwarding to the watch is supposed to take place and usually the phone will ring a couple times before the forwarding process takes place to the watch.
Yes, the watch can use the standalone phone number issued to it at time of activation for true standalone purpose but in most use case scenarios the average consumer ties a phone and the watch together. When these watches can have battery life that lasts significantly longer than what currently is capable then standalone function is secondary to normal function.
It sounds like in this case the e-sim is bad and the watch needs to be replaced.