Firms using 0800 numbers pay the UK geographic charge (plus a small premium) for incoming calls, so the system doesn't allow international callers to use 0800 numbers. (Otherwise nobody would be paying the full cost of the call).
So you need to firm the firm's 'normal' phone number. (They must have one, as 0800 calls are actually redirected to regular phone numbers). Ask here if you need assistance in doing so.