Personally, my headed paper carries address, landline (phone & fax) , email and website .
This I consider more than sufficient.  For close friends and family, they know to use mobile only if landline is  unanswered.
Most people are aware that calling mobiles is more expensive, I do not think the same applies to 0870xx, which are advertised as 'National Rate Charged', but are at 7.9p/min whereas BTs national rate can be as low as 3p, and 18866 as low as 0.5p/min.  This is where the 'confusion' for customers comes in. 
I don't think your analogy is really completely sound, if it was intended to apply to the 0870 discussions.

I have happened across this web/forum by chance; some useful chat here.

regarding mobile, I think it does depend whether it is for business or personal use;

I run my own company from home but want to use a more sterile number and therefore have an 0870 pegged through to my business line at home. the 0870 is free but then I don't get any revenue from the calls.. then again I don't really get emough calls to to really make a difference on the 2p per minute revenue (daytime).

One thing I do use though and find it very useful is good old 1571, the full call minder version as I do not publish my mobile nuumber. BT have a slight lingering attachment to old 'Cellnet' now O2, in that call minder and O2 mobiles can be linked together.... only O2 get full integration. It works like this...

If someone rings your landline and you are busy or not there , call goes to 1571. So long as you have your mobile registered to your account and turn off O2 voicemail, Call minder will ring your mobile and tell you that you have a message (you don't pay for this call), when answered you just press 1 to get the message. If you are driving (with bluetooth of course) you can use voice commands to interact with the system, listen, save, delete etc.

If you do not answer the mobile when 1571 rings you then 1571 sends an SMS to you saying you have a message.

Now, all you do is dial 1571 from the mobile and you get your messages... or if driving just use voice promts to ring voicemail and then talk to 1571.

Further... if you turn off the O2 mail and type in 1754 and send you then direct your missed calls to your 1571 call minder mailbox!

Bingo both your landline and mobile calls all go into a single mailbox and when you get a message left the sytem finds you... either at home on landline or if thats busy it rings you on mobile or texts you.

It is great, does cost you a little eachg month to BT for the call minder service, but if you use CLI and maybe another service anyway you can get a bundle price; It is v good value and for business purposes means you can have one mailbox only which is probably the best bit.

reagrds