The text messaging feature is new starting with firmware version 355B16. It uses a proprietary protocol (not "SIMPLE"). Text messages can be sent to a handset in two ways:
To enable text messaging, go to the Management Settings page on the Web User Interface of the base station.
Once this in done you will need to do a Handset Reset on the phones to activate Text Messaging.
Please be aware that this procedure clears-out all the settings on the handset and de-registers it from the DECT chain.
After the master reset you must re-register the handset to the DECT chain.
You can now send text messages from one handset to another.