If the IP address is assigned dynamically via your local network's DHCP server, you can find the current IP address on the DHCP server in the list of registered DHCP clients. The MAC address can be found on the rear of the device. If necessary, contact the network administrator for your local network.
Your DECT manager's IP address may change occasionally depending on the DHCP server settings.
SIP multicast: After every boot, the device will send a SIP multicast which can be seen in wireshark if you are connected to the same network and are in the same vlan.