we have m700 base machine and m25 handset.
we need to call extenstion to extension with out using call software server
and to register extension in base machine without domain
if you have issues with registrations, please, verify on your pbx the error that it shows when this happens (logs).
You can see it also with a sip or pcap trace.
If you need more tips please open a support ticket with a pcap / sip trace.
This is not supported and will not work. Each handset requires registration with your SIP registrar/PBX and an assigned extension.