Creates a new contact or updates an existing one based on the phone number.

Use Case: Use this endpoint to synchronize your customer data with Wax CRM. This is useful for:

• Importing contacts from your e-commerce platform, CRM, or other systems
• Updating contact information when customers update their profile
• Adding custom attributes to contacts for segmentation and personalization

Technical Details:

• If a contact with the given phone number exists, it will be updated with the provided data
• If no contact exists, a new one will be created
• Custom attributes are upserted (created or updated) individually
• Tags are added to the contact (existing tags are preserved)