A search for Phonebox direct gives a cross reference of Mobile Media Systems.

One search gives on 192.com
MOBILE MEDIA SYSTEMS LTD
22 THE ROPEWALK
NOTTINGHAM
NG1 5DT        No number

Phonebox direct
RADCLIFFE-ON-TRENT
NOTTINGHAM
NG123WL            0870 153 9131

Phonebox direct website (also known phone-box.co.uk)gives         
Mobile Media Systems
PO Box 8161
Cotgrave
Nottingham
NG12 3WL            08700 101871

Found this line on http://www.e4s.co.uk/docs/student-mobile-phones.htm
PhoneBox Direct (part of the Phones4u partnership)
so perhaps you could go via phones4u, they might have the same deals

A whois check gives
Domain name:
       phone-box.co.uk

    Registrant:
       Phonebox

    Registrant type:
       Not supplied

    Administrative contact's address:
       30 High Street
       Arnold
       Nottingham
       NG5 7DZ
If you can get a number for the address by reverse search


mycartel.co.uk gives
Mobile Media Systems
PO Box 7404,
Radcliffe-on-Trent,
Nottingham,
Nottinghamshire
NG123ZY

Phone: 0871-871 0081      

Thomweb only give 0870 
BT haven't heard of them


Don't know if any of this is any help, but at least you don't have to look where i did

Could somebody help me with GN for 08701539131 (customer services phoneboxdirect.co.uk).Saynoto0870 suggests 01270412425 but that always leads to sales.

Insane