Snom M300 and M700 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 you can send the SIP message to the account SIP URI.
In case you don't have any account configured on the base 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, on the M700 can be changed via the setting network_sync_chain_id. The M300 doesn't allow to change the Chain-ID.
Here is an example of a SIP NOTIFY sent to the DECT base with IP address 172.16.18.51 and Chain-ID 512:
NOTIFY sip:firstname.lastname@example.org SIP/2.0 Via: SIP/2.0/UDP 172.16.18.15:7089;branch=z9hG4bK001b84f6;rport From: sip:email@example.com To: sip:firstname.lastname@example.org Contact: <sip:email@example.com:7089> CSeq: 10 NOTIFY Call-ID: firstname.lastname@example.org Content-Length: 0 Event: check-sync