Snom M300,M700,M900 support the check-sync SIP event. If you have a SIP account already configured on the device you can send the NOTIFY message containing the Event: check-sync header.
If you have a SIP account already configured on the DECT base station you can send the SIP message to the account SIP URI.
In case you don't have any account configured on the base station you can send the SIP NOTIFY to the SIP URI derived from the DECT Chain-ID: sip:<chain-id>@<device_ip>. The default Chain-ID is 512, the value can be changed via the setting network_sync_chain_id.