Certificates for Lync Phone Edition TechNet article was updated Jan 7,2014 ( http://technet.microsoft.com/en-us/library/gg398270 ). The Trusted Authorities Cache now includes DigiCert and a few more 2048-bit (Key Length) Root certificates.
The January 2014 Lync Phone Edition (LPE) CU includes the updated list of trusted public certificates.
January 2014 CU for LPE (Aastra 6721ip/6725ip)
January 2014 CU for LPE (HP 4110/4120)
January 2014 CU for LPE (Polycom CX500/CX600/CX3000)
January 2014 CU for LPE (Polycom CX700 and LG-Nortel IP Phone 8540)
Prior to the January update, certificates issued from DigiCert had to be requested in the Legacy format to work with LPE. Jaap Wesselius’ blog post details this issue: http://www.jaapwesselius.com/2013/12/01/polycom-cx600-fail-to-login-after-certificate-renewal-on-lync-2013-edge-server/