Sometimesit has answered your question. Either "zero zero" or "we do not have the caller's number to return the call", when you dial 1471, means that you were called from overseas.
Registering with TPS is a sound idea but it won't block calls from overseas. (UK companies using overseas call centres have to stick to the TPS rules but firms which are actually based overseas aren't bound by the TPS rules).
If Boxtops is paying 'less than £40 p.a.' for TPS registration, I sincerely hope that the amount she's paying is actually £40 less than £40 p.a. TPS registration is FREE!
Chris