the dmr.cumbriaCQ.com DMR server now supports Private 1 on 1 calling. So if 2 of you are connected via a pi-star to dmr.cumbriacq.com server you can both use the Private call function on your radio and dial the DMR ID of the other user.
A Mobile APP called Z3DMR works very well with this
SMS is also enabled on the server too. So you can send and receive SMS messages via your radio and / or DMR APPS like Z3DMR
Setting up yout hotspot on cumbriaCQ DMR can be found here Cumbria CQ DMR Server - CumbriaCQ UK
It is recommended for all this to work correctly, to disable all other networks on the pi or figure out how to pass private calls over cumbriaCQ network using the DMR Configs. But simple option is disable all other networks.
73 M5LEE