It is believed to be strictly illegal to use your Mercury Code from other than your designated phone number. However, it may well be possible to to use it from any line on the same exchange as you. Look upon the inability to use it outside your local exchange as a security feature.
You could consider their chargecard (Mercury Calling Card) service, instead.
The answer beep can annoy customers if it occurs once the person has started speaking. In some areas, it is immediate on answer and therefore no problem. It can be removed for specific accounts on request: just ring up Mercury customer services and ask.
The beep triggers call logging/charging equipment, such as hotel switchboards and payphones, and although it is currently applied to all indirect calls via Mercury (except where the customer has requested its removal), Mercury are planning to withdraw it from all those customers that do not specifically require it.
The code is 132. Users register their calling lines with Mercury. 132 also works for data or voice calls over (BT installed) ISDN.
131/PIN service: try tone dialling 131 wait-for-tone 1234567. If the service is available you'll get a recorded announcement starting 'A test PIN code has been used to dial this number'.
For the pin-less service, dial 132. If you get a message "We are sorry, but we are unable to connect you to the Mercury network..." phone 0500 500 194 and ask about it. If you get "[SIT] I'm sorry, the number you called has not been recognised" then 132 isn't available to you.
You can also dial 0500 132 131 and follow the automated prompts.
If the phone is rented by a customer from BT, the answer is yes, provided that (a) it is in a Mercury Access Area, and (b) the customer has not asked BT to bar access to Mercury from that phone. If however the phone has been provided by BT for "public" use (i.e. nobody is paying rental on the phone, and all the takings go to BT) then, in nearly all cases BT will have barred Mercury access. If there are any cases where Mercury access has not been barred, it will be simply because someone has forgotten to do it!
For Mercury 131, try using a #. This apparently doesn't work for 132, because for that service the BT exchange stores all the digits and dispatches them to Mercury in one go.
The Mercury Communications Ltd. Price List (or Tariff Schedule
Marketing Publications Manager Mercury Communications Ltd New Mercury House 26 Red Lion Square LONDON WC1R 4HQ Voice: (0171) 528 2000 (main switchboard) Fax: (0171) 528 2377 (direct to Marketing Pubs dept I think)
In case anyone wants to know what it looks like (so they can check they're being sent the real thing) it's an A4 ring binder about an inch thick, there are several sections inside each separated by a snazzy plastic divider with a graphic symbol designed to represent the contents.