@cfcman no worries...
Below is the list that is displayed in the Voipon document I posted above. The list is in the following format: Handset model, display, phone book, recommended base station, battery life, bluetooth and finally hands Free:
- Gigaset A58H
B/W display 96x64 pixels
150 entries
See compatibility table
17 hours talk 210 hours standby
- Gigaset C59H
Colour graphic TFT display
150 entries
Any Gigaset
12 hours talk185 hours standby
- Gigaset C300H
Colour graphic TFT display
250 entries
See compatibility table
20 hours talk 300 hours standby
- Gigaset E49H
Colour
150 entries
Any Gigaset
12 hours talk 250 hours standby
- Gigaset S79H
Colour graphic TFT display
500 entries
Any Gigaset
13 hours talk 180 hours standby
- Gigaset SL400H
Colour graphic TFT display
Siemens Gigaset HC450 Intercom
500 entries
Any Gigaset
14 hours talk 230 hours standby
- Gigaset SL78H
Colour graphic TFT display
500 entries
Any Gigaset
14 hours talk 200 hours standby
- Gigaset E500H
B/W display 128x80 pixels
150 entries
E500A
20 hours talk 300 hours standby
- Gigaset A510
B/W display 96x64 pixels
150 entries
Any Gigaset
20 hours talk 220 hours standby
Unfortunately, the HC450 isn't included in Gigasets compatibility checker, no idea why! But, going by the above, the Gigaset SL400H is compatible with the HC450 door entry unit, it supports Internal calls, intercom, but i'm unsure if it supports selectable PSTN/VOIP calling on a per-call basis (it should). The Gigaset compatibility checker confirms full compatibility between the Gigaset SL400H and the N300 IP base station too.
It's available on Amazon
here. Not cheap mind, £81.99 FREE UK delivery each. Maybe order one and give it a try? If it doesn't work, send it back. :)