Hi partner,
In addition to business rules, you could also use workflow.
If my goal is: When I modify Mobile Phone, Business Phone automatically assigns the same value. When Mobile Phone is to be set to 0, the Business Phone does not automatically assign the same value.
1) Create a new workflow, check Record is created and Record fields change in the Start When column, and then click Select button. Select the data source field. Click OK.


2) Click Add Step > Check Condition, then click <condition> (click to configure), select the corresponding condition. Save and Close.

3) Click "Select this row and click Add Step", then click Add Step > Update Record, then click Set Properties. Set matching field in the corresponding field. Save and Close.


4) Click Save, then click Activate.

This effect is:
When I modify the value of Mobile Phone, save the record, refresh the page, the value of Business Phone will automatically become the value of the Mobile Phone.

When the value of Mobile Phone is changed to 0, the value of Business Phone does not change.

The drawbacks of this way is:
If the two fields are different before the workflow is created, the two fields will not be automatically synchronized without modifying the record.
These two fields can be synchronized only when the record is modified.
Hope this helps.
Best Regards,
Lu Hao