I am getting this error sending bulk sms using sand box

RingCentral.RestException: 'Response:
StatusCode: 503, ReasonPhrase: 'Service Unavailable', Version: 1.1, Content: System.Net.Http.StreamContent, Headers:
{
  Connection: keep-alive
  Retry-After: 40
  X-Rate-Limit-Group: light
  X-Rate-Limit-Limit: 50
  X-Rate-Limit-Remaining: 49
  X-Rate-Limit-Window: 60
  RoutingKey: SJC11P01
  RCRequestId: bebbb5be-5699-11ed-846f-005056bbba6c
  Date: Fri, 28 Oct 2022 08:23:05 GMT
  Server: nginx
  Content-Length: 107
  Content-Type: application/json
}
Content: {
  "errorCode" : "AGW-110",
  "message" : "Internal Server Error. Consult RC Support.",
  "errors" : [ ]
}