Lists the extensions on your account, ordered by extension number ascending. Only enabled extensions are returned unless you ask for all of them. Each row includes the extension's SIP password, so treat the response as a credential, and carries its nested voicemail, devices and extension_users records. Every field is nullable. hold_music comes back as a sound reference (an object such as {"type": "recording", "id": "…"}), or null when not set, in the same form a create or update takes.