Quick Update [written on 18 August 2014], after installing Lync August 2014 Commutative Update, Microsoft decided to always show the mobile number of all people from Lync Mobile Client : http://support.microsoft.com/kb/2967486  

Hi, for those using Lync 2013 on their mobiles and wondering why the contact card is not showing the mobile number, here is the trick.

By default, All your Lync contacts are marked as (Colleagues) relationship. You can view this by right clicking any contact on your (Lync 2013 desktop ) contact list and click “Change Privacy Relationship”.

Lync 2013 mobile, will show mobile number for your contacts if and only if you both mark your self as (Work group) privacy relation ship.

Example

I have in my contact list a contact named John. When i use my Lync 2013 mobile, i can only see his work number and not his mobile number.

To solve this, i went to my desktop Lync 2013 client, i right clicked his name in my contact list > Change Privacy Relationship > Work group

I then called John and asked him to add me to his contact list and set the privacy relationship to (workgroup).

Now, when i sign in to my mobile lync 2013 client, i can see his mobile number in the Lync mobile contact list.

Note: I guess this is due to protect privacy issues, and i didn’t find any way to change this behavior so that everyone by default can see other’s mobile numbers from Lync 2013 mobile