I had a 2.4ghz set from Seimens that handled 2 lines with multiple handsets, but the quality was crap and inside of a year the entire setup was unusable. When I tried to send it back to seimens, they said "nope, discontinued, no repair, no replacement, kindly play hide-and-go-fuck-yourself".
It'll be a cold day in hell before I buy another seimens product, and for obvious reasons I'd counsel you against it as well.
If you do find a good setup though, let me know. I've been using a couple panasonic 900mhz phones which more or less do the job, but not as well as I'd like.
Edit: Hm... someone posted an URL above and that site has a multiline section. Maybe something in there will work for you:
http://www.cordless-phones-america.c...sp?subcat=CPML