okay, as far as I understand:
1.) regardless of having registered an external phoneline, phones will be set up. Any MD has an integrated phone, which will be setup and registered automatically.Those phones could for example be used as an intercom. If you set up an external phoneline, then your "intercom" becomes a real telephone.
2.) if you want to use your gigaset together with LMCE, you need to set it up manually in LMCE first. As far as I know, there is no template yet for this specific phone.
So in order to set up your phone go to webadmin->phones->add device. From there you have to pick a template- either your phone is listed, or you use "Generic SIP softphone". This wil create the proper phone for you. You will then have a phonenumber and a password.
3.)if you have done the above steps, you open the website for your gigaset and there you select an ip-connection (don't know with yours, but with mine I can define up to six connections) With this connection you use the phonenumber as both number for the extension as well as username.
as for notifications, you probably need to do this in two steps:
webadmin->telecom->call routing. There you can define what phones will ring for at home, do not disturb, away and sleeping
the other one is to edit the settings for the phoneline: once you have successfully registered a line with automatic configuration (for me sipgate.co.uk did actually work) you can click on settings. There you can change settings for armed-at home, armed-away, entertaining, sleeping and unarmed-athome. So this will give you proper notifications and you can select, that for example when watching TV or movie, you send your callers to a specific extension or straight to the mailbox
I hope this will help you along a bit. But first I guess you have to get your asterisk running properly, or did you happen to fix it now?