AudioCodes 420HD – Failed to validate server

AudioCodes 420HD can be a good replacement for Lync Phone Editions, at least regarding its price. However, don’t expect to have all the features that exist in the Lync Phone Editions. In addition, AudioCodes has already promised to enable Better Together over Ethernet to 420HD, 430HD and 440HD in a future firmware release.

The provisioning of these phones is different, but I strongly recommend that you prepare the same requirements as needed by Lync Phone Edition Phones. Jeff Schertz has a great article on how to configure the requirements:

Configuring Lync Server for Phone Edition Devices

Now that we have configured the requirements, let’s go back to the phone. 420HD supports two methods of authentication, one by using Sign-in address:


Another by using Phone Extension:


To change the Sign-in method, we use the option Switch sign-in method that is available in the Sign in menu:


After setting the method and trying to sign in, I got the error saying that Failed to validate server:


In this case, I thought that it had to do with certificates and that the phone didn’t trust Lync Server certificate issued by the internal CA.

The phone firmware version was updated to


We can install the internal root CA in the phone. The first step is to go to Configuration>Security>Security:


In this page we can see that the CA certificate is empty:


To load the certificate, select Choose File, then select the proper certificate (*.cer). After selecting we need to Load the certificate.


Almost done. Just reboot the phone and try to sign in again: