ServicePointManager.ServerCertificateValidationCallback is not invoked

I have the following code in Android, iOS and WPF application layers.

ServicePointManager.ServerCertificateValidationCallback = delegate(object sender, System.Security.Cryptography.X509Certificates.X509Certificate pCertificate, System.Security.Cryptography.X509Certificates.X509Chain pChain, System.Net.Security.SslPolicyErrors pSSLPolicyErrors)
            {
                              return true;
            };

In WPF the callback is invoked but in iOS and Android I see no invocation of the callback.