I can confirm that, as of a year ago but I don't expect it's changed, call sign uses the same voicemail box for both lines, be it the free 1571 or the paid for Call Minder.
You can get boxes that plug in and split the line based on the incoming ring. But note that CLIP on coutgoing calls is always your main number.
VOIP may be your friend though; there are plenty of suppliers who will give you a free geographic inoming number
see here, with or without voicemail, including sipgate IIRC, and outgoing calls can be had from free. (I currently use voipcheap.com for free uk calls, but there are loads of brands by the same company that may be better for your calling patters - see
here for comparisons. All you need is a proper SIP based adapter that you connect to your router, and plug your phones in. NOT anything USB or skype based - thats a closed incompatible system, SIP is open.
Once you get a voip incoming number, point your 08* at it if you want. You can of course connect up from anywhere to receive calls - there are even wifi cordless phones that do it all in one - just stand in a wifi hotspot anywhere in the world..