Phone data, including extensions, is stored in the communicationItems child array on each contact record. Each item contains a communicationType (such as Direct, Cell, or Home Fax), a value (the phone number), and an extension field.
You can retrieve this data using these endpoints:
GET /v46release/apis/3.0/company/contacts
GET /v46release/apis/3.0/company/contacts/{id}
The first endpoint retrieves all contacts and supports filtering by communicationItems. The second endpoint retrieves a specific contact by ID and includes communicationItems in the response. Currently, there is no dedicated API endpoint for phone number extensions. This feature request is to add functionality that extracts the extension value from the response and exposes it as a connected field.