Help - Manual audit to populate CMDB


I am looking into the possibility of implementing the Cherwell CMDB module into the organisation I work for as the SACM Manager. 

Auto discovery is often used in support of the verification and audit activity of Configuration Management .

We currently have SCCM and Lansweeper to carry out auto discovery of CI's on the network, however we have a number of mobile phones and Tablets that are not trackable on the network so using a auto discovery tool would not be useful.  

I would like to know the best way to identify or discover mobile devices that are not trackable on the network .

I was thinking of manually contacting the various locations we have throughout the UK (we have approximately 160 locations) and simply asking the question how many mobile phones and tablets you have onsite? or sending an email to the various locations and asking the question in a email.

Any help would greatly be appreciated