BST,
The only ISP that I am aware of, that has this set up, is AOL.
It is called their "AOL Broadband Gold Plus" package. I enquired about this a while back. Here's a cutting from their reply to my e-mail.
When on the AOL Broadband plan if you wish to connect to AOL using the freephone access numbers also, you can select the AOL Broadband Gold Plus plan which is £ 31.99 per month. �31.99 Broadband Gold Plus plan which means you will have unlimited access on Broadband and also on narrow band (56k modem). If you travel a lot, or just want to be able to use the Internet when you're away from home, take a look at AOL Broadband Gold Plus. For just �31.99 a month you get your AOL Broadband Gold connection at home plus you're able to use AOL dial-up away from home.
Hope this helps