This response is based on the information provided.  We suggest you discuss with your contracting team, program manager and/or legal department as appropriate. 

You identified the correct FAR citation, FAR 45.000(b)(5).   What does the program team want to do?  There is nothing to preclude you from keeping mobile phones as accountable items.  We believe it would make sense to have the mobile phones identified as GFP since the phone is portable unlike a landline phone.  When an contractor employee leaves the job, you will want to transfer the mobile phone to the person replacing him/her.