How to sync Mobile number field from Windows AD to Entra ID?

I need to take the contents of the “Mobile” field within Windows AD user records, i.e:

…and synchronise this across to the “Mobile phone” field within the corresponding user record in Azure Entra ID i.e.:

We already sync other user data via an Azure AD Connect service, but despite creating a synchronisation rule to map the fields, the field is not updated within Entra ID, and I cannot see any errors suggesting the cause.

Please can someone advise whether this action is possible and how the sync should be configured?