A check of the terms and conditions reveals that Seriously is part of A-Standard Europe Ltd, company number 3705228.

From a whois of seriouslyinternet.com, the address is:
A-Standard Europe Limited
7/10 Chandos Street
Cavendish Square
London
W1M 9DE

The 0871 is listed on the whois.

192.com gives the same address with a different post code: W1G 9DQ Unfortunately, there is no telephone number listed.

oxygenuk wrote on Apr 10^th, 2006 at 5:44pm:

No, that would be far too straightforward. 

0871 numbers are charged at a whole range of different rates between 5p and 10p per minute or per call. The next few digits are needed to identify the rate charged.

Numbers beginning with 0871 223 are charged at BT's g7 rate, which is 10p per minute at all times. The TalkTalk Price List indicates that g7 is charged at 10p/min at all times on its tariffs. If you use another provider, they may charge a different rate. Mobile providers usually charge higher prices for calls to these numbers.