Microsoft Skype for Business Server 2015

Hello Renato, Mobile clients should not be connecting to the AV virtual server to login successfully but instead to the reverse proxy virtual server on port 443 which (in single BIG-IP scenario) will in turn load balance traffic to port 4443 on the Lync FE's. To get mobile clients working through F5 using the iApp the reverse proxy section is key to getting this working ("thick" clients do not necessarily need reverse proxy to login and, depending on configuration/client/location, may not even be going through your BIG-IP currently).

Here is a useful technet article that goes through how lync mobility works: https://technet.microsoft.com/en-us/library/hh690030(v=ocs.15).aspx.

Let me know if i can help further once you attempt via reverse proxy Virtual Server.