Yes, you will be able to use NumberSync for wearables for your calls; however, the messaging experience will be limited. AT&T Messages Backup & Sync (AMBS) does not sync your text messages with your iPhone nor can you access iMessages from the smartwatch.
For the best possible experience while using the NumberSync for wearables service with iPhone, you’ll need to turn off iMessage® in order to receive all text messages on the smartwatch. On your iPhone, go to Settings, Messages, iMessage and select OFF. When not utilizing NumberSync for wearables, you can choose to turn iMessage back on.
To learn more about the NumberSync service with an Apple Watch Series 3 with cellular, go to the Apple Watch (Cellular) tab.