When you have pure Lync common area phone - using LAN not USB - its sign-in fails to the "Internal error". On the IIS's logs I can see the following lines:
2013-11-14 01:08:41 10.19.71.47 POST /WebTicket/WebTicketService.svc/mex - 443 - 10.11.12.13 OCPhone/4.0.7577.4372+(Microsoft+Lync+Phone+Edition) 200 0 0 56
2013-11-14 01:08:42 10.19.71.47 POST /WebTicket/WebTicketService.svc/pin - 443 - 10.11.12.13 OCPhone/4.0.7577.4372+(Microsoft+Lync+Phone+Edition) 401 0 0 14
2013-11-14 01:08:58 10.19.71.47 POST /WebTicket/WebTicketService.svc/pin - 443 - 10.11.12.13 OCPhone/4.0.7577.4372+(Microsoft+Lync+Phone+Edition) 500 0 0 15016
Do you have a good idea from where to look this 500 HTTP error?
.-Petri-.
Petri