Hola Antonio,
Te recomiendo que le des una mirada a los siguientes artículos sobre federación de dominios.
http://blog.asirsl.com/Lists/EntradasDeBlog/Post.aspx?List=9cdf87d5-c7fe-4376-998f-c7b0e3ae1a52&ID=371&Web=2947a181-12bd-4c3f-b12a-19ac0ce9566b
https://technet.microsoft.com/es-es/library/jj205335(v=ocs.15).aspx
https://technet.microsoft.com/es-es/library/jj204800(v=ocs.15).aspx
https://technet.microsoft.com/es-es/library/gg520966%28v=ocs.15%29.aspx?f=255&MSPPError=-2147217396
add additional SIP domains in already existing Lync 2013 deployment
http://exchangequery.com/2015/04/04/steps-to-add-additional-sip-domains-in-already-existing-lync-2013-deployment/
http://social.technet.microsoft.com/wiki/contents/articles/30621.steps-to-add-additional-sip-domains-in-already-existing-lync-2013-deployment.aspx
Espero sea de gran ayuda.
Saludos