We have installed the add customers from admin addon and use the COD payment module, obviously changing the text within it, as a telephone payment option. It means we can process telephone orders through the site and use telephone payment as the payment option.
It also means customers have a site login to use and a discount coupon burning a hole in their pocket which is never a bad thing!
If you then install the master password addon you will be able to access accounts for your customers by email address and have their address details to hand when they ring.

Hope it helps.