Linksys / Cisco SPA8000 VoIP gateway. Multiple channels/lines with a single SIP account.
It’s not possible, but here’s a great solution.
The SPA8000 is designed to be fed by a VoIP PBX. It’s not designed to be fed by a multi-channel SIP trunk from an ITSP. The scenario for which the SPA8000 was designed is a corporate environment where you want to support a number of analog devices, but use your SIP PBX. e.g. say you want to have 8 DECT cordless phones on your system, without having lots of ATAs.
The solution: put an Asterisk PBX in between the SPA8000 and your ITSP.
The perfect candidate for this job is the Atcom IP01 Blackfin based device. They’re less than £100.
You should then set each line of the SPA8000 to connect to an extension (701, 702, 703 etc.) of the Asterisk PBX. You can set up the hunt/ring groups from the Asterisk PBX.

Note: firmware 6.1.3 adds support for “Trunks” and “Trunk Groups”. I beleive this should achieve what is desired, however I like the flexibility of the Asterisk box, and the customer is enjoying the voicemail features provided by it.