If you have an lte watch connected to a watch plan the watch should use the host phone number.... my watch 5 acts independently of host line and uses the separate watch number unless connected to phone through Bluetooth
Thank you for reaching out. I can certainly understand your concern with number share not working on your Galaxy watch 5 LTE. I would recommend reaching out to your carrier directly as number share is a carrier feature. Furthermore, I would also verify with your carrier that your device is set up to have the number share feature activated, most watches are activated with their own line and then number share is set up.
