Our firewall was configured as per Google’s documentation to allow the Google Hangouts nee Talk client and the Android Google Hangouts app to connect. but to no avail.
With a little bit of troubleshooting I found that an additional port needed to be opened too. So here’s the firewall ports you need to open for Google Talk:

• 80 TCP

• 443 TCP

• 19302 – 19309 TCP and UDP

• Additional Ports for various clients

  <td>
    Clients
  </td>
  

  <td>
    XMPP clients
  </td>
  

  <td>
    Android phones
  </td>
  

  <td>
    XMPP federation
  </td>
  

  Ports

  5222, 5223, 5224

  5228 and 5229

  5269

This information was taken from  Optimize your network for Hangouts

I hope that helps some of you out there.

Mike