The majority of web-based internet companies do not have phone numbers, ourselfs included. Support by helpdesk is much more cost-effective and efficient means of support.
Of course, anyone can always request us to phone them for accounts, sales and general information about our services and company, but we refuse to provide technical support by telephone.
Have you attempted requesting them to call you? If they are ignoring your emails and if your only alternative is to find out and phone their office telephone number, this is what you should do.
http://whois.xreg.co.uk/?domain=seem.co.uk
Their Nominet IPS Tag appears to be SEEMINTERNET, as they are a member of Nominet UK (the uk naming authority) phone them up on +44(0)1865 332211 (9-5 office hours I believe) explaining that you are unable to get in contact with your domain's TAG holder and you need some modifications made to your domain.
They should give you their phone number (Nominet constantly do this, even though it is against the data protection act) as they do not wont to deal with the issue directly.
Let us know what happens. Out of interest, why do you need to phone them directly?