In order to connect with other Lync platforms the other organisation will need to add your FQDN (Fully Qualified Domain Name) to their platform.
The Cobweb Hosted Lync 2013 platform is already enabled for federation with other organisations.
Enabling Federation with Cobweb
Full instructions on how to configure federating with another platform are explained here.
The settings required are:
Domain name: Yourdomain.com
Access Edge Service: access.hs20.net
This needs to be completed for every domain name that uses Lync.