First guess for someone that persistent would be a calling centre for someone. Some outbound calling centres use automated dialling services that guess when to dial and when an operator will be free to pick up your answered call. If the system is set too agressively you get a lot of nuisance calls.
Might be worth a call to Ofcom about it.
More information, including stuff on companies recently fined by Ofcom for this
here.
Found more info specifically on numbers giving a message about ICT when called back
here