I personally have the Gigaset C455IP & multiple handsets in two locations, and have had a number of previous generations of Gigaset, so have been investigating integration with this device already, but haven't concluded what's the right thing to do, yet.
It'd be wonderful if these devices already supported HTTP-type callout on significant events, like ring & answer & on-hook, but they don't! Siemens publish a lot of the source code to these bases, because they're based on GPL'ed Open Source, but that'd be quite a commitment for H2M8, at this stage. So I'm posting these as suggestions to Siemens, awaiting their response.
But also, these devices support the Jabber Instant Messaging protocol, so it should be possible to do a lot of integration through such an Instant Messaging interface. Again, this would be rather a commitment from H2M8 at this stage, but it's something worth investigating as part of our general investigation into Instant Messaging interfaces.
Please update us all if you find anything that'd work already, with very little new coding! I'll be the first to use it!
-- thanks, Alistair