Question Samsung Galaxy S5 Forwarding to Unknown Number

I have an unlocked Samsung Galaxy S5 handset. I believe it was originally a Vodafone UK locked handset but I now use it on the Three UK network.

I have checked my first bill and noticed that various calls to an unknown number. In addition I haven't received any voicemail messages. I am assuming that the phone is forwarding calls to this 0330073XXX number.

27 May 15:56 0330073XXX National Landline (Call Fwd) 00:01:00 Included 0.000

When I check all my diverts and calls forwarding on my handset, it shows my correct voicemail number.

When I call 0330073XXX I hear my voicemail message?

Is this a handset, app or provider issue?