Phone numbers aren't displaying correctly

Hello everyone,

we've got the problem that contact names from internal colleagues aren't displaying when they are calling from external (via mobile phone) - only the telephone number is shown. After you accept the call, then the name will be resolved and you see who is calling. 

But this is not happening all the time, sometimes the name of the colleague is shown before you accept the call.

Further information:

+ Mobile phone information are stored in AD (format E.164 -> example +49 (999) 99999 )

+ Lync Server 2010, OS Windows Server 2008 R2

+ Clients - Lync 2010 x64/ Lync 2013 x64

Does anyone have an idea why this is happening or what the problem is?

Thanks in advance!

Best regards,

Hermann