After you have completed the DECT registration process and added the IPEI of your handset to the M300/M700, you can assign multiple SIP accounts to those handsets.
In example above (snomM700) you can see that I have assigned SIP extensions 5658 and 5695 to the handset 02556FCE35 (Index #3)
If the SIP registration is not working then select the corresponding extension and start the SIP registration.
In example above (snomM700) you can see that I have assigned SIP extensions 3002 and 350 to the handset 025482B3D6 (Index #5)
On the handset press the Menu key → Settings → and press the Menu key to select the "Line" option.
You should now see the available SIP extensions for that handset.
All listed extensions are active for inbound calls. To make an outbound call you need to select the primary SIP extension/line in this menu.