There can be many reasons why Lync cannot contact external contacts.I have put a checklist together on the most common thing you should check to successful connect to federated partners.
Select “Access Edge Configuration” and make sure that “Enable Federation…” has been checked. Also make sure that you either use an internal DNS server or have a host file with the names of the frontend/directors servers.Even if you are setting Lync up for the first time or you have done it several times, you might end up making a mistake in the configuration or forget something in the setup.So use this guide to check if you have configured everything correct – which you might not have, since you are looking at this blog Select properties on the Edge Server Pool Make sure that the port on the Access Edge service has NOT been set to 5061. Make a record on the three names that have been defined – you will check them later on. Select the “External Access Policy” – Check that the users is allowed to communicate with external contacts. COM Check the firewall, and that you have created the right port openings.One important case your application needs to account for if it may be deployed across more than one pool of servers is receiving presence updates from different pools of Front End servers.You can often get away with ignoring this effect, but you risk running into strange and seemingly inexplicable behaviour once in a while.Let’s create a simple application from scratch with two buttons: one to update Skype for Business status to Busy and one to update it to Available.