Hi
The functions that are available on your Apple Watch when it is away from your iPhone depend on various factors including:
- Your Apple Watch model:
- The features of your carrier service plan:
- The type of connectivity that is available to your Apple Watch at the time:
About Bluetooth, Wi-Fi, and cellular on your Apple Watch - Apple Support
Features and requirements that apply under various circumstances are detailed here, including within footnote #2 in respect of calls:
How to use your Apple Watch without your iPhone nearby - Apple Support
In respect of call-related issues when using the built-in cellular feature, I suggest that, in the first instance, you contact your carrier for assistance.
If the issue persists, then, depending on your carrier's advice, I suggest that you next contact Apple Support (remote diagnostics and/or mail-in service may be available, if required), make a Genius Bar reservation or visit an Apple Authorised Service Provider for assistance: