Elimine el loop del login activity comentando la parte donde se llama el softphone activity:

  private void llamarSoftPhoneActivity() {
        SharedPreferences.Editor editor = sharedPreferences.edit();

        Email = editText_Email.getText().toString();
        int atIndex = 0;
        atIndex = Email.indexOf('@');
        String Username = Email.substring(0,atIndex);

        editor.putString("Usuario", Username);
        editor.putString("Email", Email);
        editor.putString("Contrasena", editText_Contrasena.getText().toString());
        editor.putString("IMEI",aIMEI);
        editor.commit();

        finish();
        //Intent intent = new Intent(this, PnSoftPhoneActivity.class);
      //  startActivity(intent);
    }
Elimine el loop del login activity comentando la parte donde se llama el softphone activity:

  private void llamarSoftPhoneActivity() {
        SharedPreferences.Editor editor = sharedPreferences.edit();

        Email = editText_Email.getText().toString();
        int atIndex = 0;
        atIndex = Email.indexOf('@');
        String Username = Email.substring(0,atIndex);

        editor.putString("Usuario", Username);
        editor.putString("Email", Email);
        editor.putString("Contrasena", editText_Contrasena.getText().toString());
        editor.putString("IMEI",aIMEI);
        editor.commit();

        finish();
        //Intent intent = new Intent(this, PnSoftPhoneActivity.class);
      //  startActivity(intent);
    }