In the SIP configuration in the KWS300 I have
Local Port: 5060
Transport: UDP
Server: MIPU IP address
Register each endpoint on separate port: un ticked
Send all messages to current registrar: un ticked
Registration expire(sec: 3600
Max forwards: 70
SIP type of service (TOS/Diffserv): 96

The rest is in default.

Under Users I have

DECT Settings
IPEI: IPEI number from the handset
Access code: Blank
Standby text: What you want to display on the handset eg 100 Warehouse

SIP Settings
Username / Extension: SIP extn number eg 100
Domain: Blank
Displayname: 100 Warehouse
Authentication user: URI from the SIP extn eg 100
Authentication password: SIP URI Password eg 100
Disabled: un ticked

That is about all of the settings.
Make sure you can log a SIP handset onto the system as the SIP extension you are trying to make the wireless handset first to make sure all is OK with the system/software/MIPU firmwares etc etc

Let me know how you get on.

Regards
Carl