I've turned it off - not that I ever turned it on.gg - if those calls are "real" - I suspect that they are probably the result of an auto-dialler system / marketing.
Similar to when one gets a landline call and there's no-ne there - ie the auto-dialler rang the number but the call wasn't picked up by a "customer service person" in the call [scam] centre.
Just ignore them / turn off the voicemail system ... and don't worry about it.
Bonkers that giffgaff / 02 whatever can't delete zero length messages - or at least include caller ID - or if spoofed, once again, delete them...gg - if those calls are "real" - I suspect that they are probably the result of an auto-dialler system / marketing.
Similar to when one gets a landline call and there's no-ne there - ie the auto-dialler rang the number but the call wasn't picked up by a "customer service person" in the call [scam] centre.
Just ignore them / turn off the voicemail system ... and don't worry about it.