If the DNS settings are not configured correctly in your LANCOM router, the router will not be able to correctly resolve the DNS address of the SIP registrar (e.g. tel.t-online.de).In this case you should check the DNS settings of your LANCOM router in the menu IPv4 -> DNS. If DNS requests are explicitly forwarded to specific remote sites, you should check that the Forwarding settings are configured correctly.
Information on how to configure DNS forwarding is available in this Knowledge Base document