Can a company (with or without switchboard) sign up?
Yes, just tick the 'company' box when registering. The payment conditions and how the service works is exactly the same as for residential customers. You will be able to view your itemised VAT invoice online as usual.
If you wish to make calls through your switchboard (PABX) you should contact the company who have installed your switchboard, or check the manual, to have the 1899 prefix programmed so all calls will automatically go via 1899. Alternatively you can dial manually by over riding the current prefix code (usually ##) followed by 1899 and the telephone number you wish to call.
Please do be careful adding generic company, university and shared accommodation telephone numbers to your account as anyone with access to the shared telephone system will be able to use 1899 through your account.