Having done a google search it came up with a geographical fax number of 0207 744 0099.  Therefore, try 0207 744 0098 (one consecutive number less) it is definitely Siemens as I've just rung it but the office is shut now so try tomorrow or tuesday and see if its the correct department, or if they'll transfer you anyhow.

If it is the wrong department ask them for the extension number and if they can transfer you.  If they give you the extension number then try 0207 744 xxxx (where xxxx is the extension number).  I've actually done this to a few companies (not siemens though) and its worked.

If it the correct number, can you post it here so that we can add it to the database.