Get SIP Registration

curl --request GET \
  --url https://api.ultravox.ai/api/sip/registrations/{registration_id} \
  --header 'X-API-Key: <api-key>'

{
  "registrationId": "<string>",
  "created": "2023-11-07T05:31:56Z",
  "username": "<string>",
  "proxy": "<string>"
}