Gigaset DECT SIP base and multiple concurrent calls

[Dogbreath]

Using Gigaset N510IP DECT unit. It's a SIP client from the Mitel's point of view, programmed with two SIP extensions.
It has six handsets. The DECT handsets offer the user a choice of 'line' [ie, SIP extension] to use when dialling out, but there are two problems:
- It doesn't tell you if the line is in use or not
- It lets you dial out when a line is busy.
If you try to dial out on a busy line, the 3300 interprets this as an attempt to transfer the call, hilarity ensues, usually involving two external calls being conferenced together.

So, is there any way to allow two concurrent calls from a SIP extension on the 3300?
For anyone using Gigaset DECT kit, can you make them prevent concurrent calls?

I guess the problem here is, I'm expecting phone system-like features of the Gigaset, but it's not a phone system.

[bluewhite4]

Having no direct knowledge of the N510...

Is it possible to 6 separate SIP accounts on the base? And then restrict each handset to 1 account each? You would need an MCD license for each, but it would effectely make each handset its own.

This is what we've had to do for multiple handsets off of a single 5610 base.