How to Send SMS from C# and .NET with the Grupo Tecnophone API

Send an SMS with C# and HttpClient

The following console program sends an SMS and processes the response:

using System.Net.Http.Headers;
using System.Net.Http.Json;
using System.Text.Json;

var token = Environment.GetEnvironmentVariable("GTP_SMS_TOKEN")
    ?? throw new InvalidOperationException("GTP_SMS_TOKEN is not set");

using var http = new HttpClient { Timeout = TimeSpan.FromSeconds(30) };
http.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", token);
http.DefaultRequestHeaders.UserAgent.ParseAdd("MyCompany/1.0");

var payload = new
{
    to = "+525512345678",
    body = "Your verification code is 482913. It expires in 5 minutes."
};

using var response = await http.PostAsJsonAsync(
    "https://api.grupotecnophone.com/prod/v1/sms/send", payload);

var json = await response.Content.ReadAsStringAsync();
using var doc = JsonDocument.Parse(json);
var root = doc.RootElement;

if (response.IsSuccessStatusCode
    && root.TryGetProperty("status", out var status)
    && status.GetString() == "success")
{
    Console.WriteLine($"Sent. SID: {root.GetProperty("sid").GetString()} - segments: {root.GetProperty("num_segments").GetInt32()}");
}
else
{
    var code = root.TryGetProperty("error", out var error) && error.ValueKind == JsonValueKind.Object
        ? error.GetProperty("code").GetString()
        : $"HTTP_{(int)response.StatusCode}";
    Console.WriteLine($"Not sent ({code}): {json}");
}

Send SMS to multiple recipients with C#

To send bulk SMS from .NET, wrap the call in a class and reuse the same HttpClient. That way you keep the sid of each message and separate the errors.

using System.Net.Http.Json;
using System.Text.Json;

public sealed record SmsResult(bool Ok, string? Sid, string? Error);

public sealed class GtpSmsClient
{
    private const string Endpoint = "https://api.grupotecnophone.com/prod/v1/sms/send";
    private readonly HttpClient _http;

    // Pass in an HttpClient already configured with the token, User-Agent and timeout
    public GtpSmsClient(HttpClient http) => _http = http;

    public async Task<SmsResult> SendAsync(string to, string body, CancellationToken ct = default)
    {
        using var response = await _http.PostAsJsonAsync(Endpoint, new { to, body }, ct);
        var json = await response.Content.ReadAsStringAsync(ct);
        try
        {
            using var doc = JsonDocument.Parse(json);
            var root = doc.RootElement;
            if (response.IsSuccessStatusCode
                && root.TryGetProperty("status", out var s) && s.GetString() == "success")
            {
                return new SmsResult(true, root.GetProperty("sid").GetString(), null);
            }
            var code = root.TryGetProperty("error", out var e) && e.ValueKind == JsonValueKind.Object
                ? e.GetProperty("code").GetString()
                : null;
            return new SmsResult(false, null, code ?? $"HTTP_{(int)response.StatusCode}");
        }
        catch (JsonException)
        {
            return new SmsResult(false, null, $"HTTP_{(int)response.StatusCode}");
        }
    }
}

// Usage: go through the list at a moderate pace and keep the sid of each message
var sms = new GtpSmsClient(http);
string[] recipients = { "+525512345678", "+528112345678" };

foreach (var number in recipients)
{
    var r = await sms.SendAsync(number, "Your order has shipped. Thank you for your purchase.");
    Console.WriteLine(r.Ok ? $"{number}: {r.Sid}" : $"{number}: error {r.Error}");
    await Task.Delay(200);
}

In ASP.NET Core, register the class with builder.Services.AddHttpClient<GtpSmsClient>() and configure the token, User-Agent and timeout there. For high volumes, queue the sends in a background service. Without coding, you can also send bulk SMS from Excel or CSV.

Common errors

On 400 errors the response includes an error object with code, code_num and message.

Best practices

• Set a timeout and log each response with its sid.
• After a network error with no response, do not resend blindly: check first so you don’t duplicate the message to the user.
• Retry only 500 errors, with progressive backoff and a maximum number of attempts.
• Queue bulk sends instead of sending them within a web request cycle.
• If the content is sensitive, enable connection encryption; the API documentation describes the RSA-OAEP encryption format.

See also the SMS API for Mexico, the SMPP connection and the guides for PHP, Laravel, Python, Node.js, Java and cURL. If you need test credentials, contact us.