The REST API is available for sending messages

conststring username = "foo";
conststring password = "bar";
var authorization = Convert.ToBase64String(Encoding.ASCII.GetBytes($"{username}:{password}"));

var client = new HttpClient {
    BaseAddress = new Uri("https://rest.ecall.ch/api/"),          
    DefaultRequestHeaders = {
        Authorization = new AuthenticationHeaderValue("Basic", authorization)
    },
};

const string message = @"
{ 
    ""channel"": ""sms"",
    ""from"": ""0041XXXXXXXXX"",
    ""to"": ""0041XXXXXXXXX"",
    ""content"": {
        ""type"": ""Text"",
        ""text"": ""Hello eCall World"" 
    }
}";

await client.PostAsync("message", new StringContent(message, Encoding.UTF8, "application/json"));

curl -X POST https://rest.ecall.ch/api/message \
        -H "Content-Type: application/json" \
        -u "username:password" \ 
        -d \   
        "{
           \"channel\": \"sms\",
           \"from\": \"0041XXXXXXX\", 
           \"to\": \"0041XXXXXXX \",   
           \"content\": { 
               \"type\": \"Text\",  
               \"text\": \"Hello eCall World\" 
            }    
        }"

$url = 'https://rest.ecall.ch/api/message'

$username = 'foo'
$password = 'bar'
$authorization =[System.Convert]::ToBase64String([System.Text.Encoding]::ASCII.GetBytes("$($username):$($password)"))

$headers = New-Object "System.Collections.Generic.Dictionary[[String],[String]]"
$headers.Add("Content-Type", "application/json")
$headers.Add("Authorization", "Basic $authorization")

$body = "{
`n    `"channel`": `"sms`",
`n    `"from`": `"0041XXXXXXX `",
`n    `"to`": `"0041XXXXXXX `",
`n    `"content`": {
`n        `"type`": `"Text`",
`n        `"text`":`"Hello eCall World`"
`n    }
`n}"

Invoke-RestMethod -uri $url -Method 'POST' -Headers $headers -Body $body | ConvertTo-Json

String url = "https://rest.ecall.ch/api/message";

String username = "foo";
String password = "bar";
String pair = username   ":"   password;
String authorization = Base64.getEncoder().encodeToString(pair.getBytes());

String data = """
{
     "channel": "sms",
     "from": "0041XXXXXXX ",
     "to": "0041XXXXXXX ",
     "content": {
          "type": "Text",
          "text":"Hello eCall World"
     }
}
""";

HttpClient client = HttpClient.newBuilder().build();
HttpRequest request = HttpRequest.newBuilder()
  .uri(URI.create(url))
  .header("Content-Type", "application/json")
  .header("Authorization", "Basic "   authorization)
  .POST(HttpRequest.BodyPublishers.ofString(data))
  .build();

client.send(request, HttpResponse.BodyHandlers.ofString());