I have added a custom field called "Member Number". When I use the "Bulk Import Contacts using a JSON payload", which includes the "Member Number" (it is also in the column fields list), the end point updates the field "Phone Other" with the member number. Many other custom fields work fine, but why does this one not update the custom field?
As I mentioned in my reply to your other topic, custom fields are not available through the bulk import process in our V3 API.
If you are working in our older V2 API, you cannot use custom named custom fields as the V2 API can only see and use custom fields that follow a very specific naming convention (such as "Custom Field 1" for example). This is because our V2 API was designed to work with our older contacts system (which did not support custom named fields).
Please let me know if you have any other questions!