It is now possible to bill directly to most major credit cards, ie. you don't personally require a residential line to apply for a card. BT Chargecard can be reached on 0800 345 144
Apparently BT have not yet put the infrastructure in for direct-dial from any Cellnet phone, though operator calls are possible. Funny thing is, you can't make even operator calls from Vodafone phones, even though you can supposedly make them from any country in the world with a BT Chargecard.
On your bill, you are told what number you called and how much it cost, but not where it was made from.
3 levels of service.
Access is via any phone line on a 0500 number (phone on 0500 100 505 for more info). There is a surcharge for operator connected calls, but if you had to do this because the BT phone wasn't capable of DTMF, there is no surcharge.
If you have a Mercury account you can have the Calling Card bill integrated with that. If you don't, or you choose not to, you can still have a card and be sent a seperate bill. The bill includes full itemisation of both called and calling number. The Mercury card is cheaper than BT's for UK calls.
Mercury's calling card number cannot be directly called from an Orange
phone. To use it, you must dial the Orange operator and ask them to connect you. There is a fee levied for this.