curl --request PUT \
  --url https://api.open.cx/phone/sip/agents/{phone_agent_id}/outbound-trunk \
  --header 'Content-Type: application/json' \
  --data '
{
  "trunk_id": "<string>"
}
'

{
  "statusCode": 123,
  "message": "<string>",
  "error": "<string>"
}