Phones such as the iPhone do not recognise giffgaff and set the voicemail key to O2's voicemail number instead. You have to dial giffgaffs voicemail number, 443, yourself.
To turn voicemail on dial 1616 To turn voicemail off dial 1626 To access voicemail dial 443
To change how long until voicemail answers a call **61*443 *10*amount of seconds# followed by SEND Amount of time in increments of 5 secs maximum 30
To change your voicemail outgoing message call your voicemail (443) then when it answers key * to access the menu and follow on from there.
To access your voicemail from another phone or landline you must first have set your voicemail PIN. Call your number, wait for voicemail to answer and then hit *. You will then be asked to enter your PIN and after doing so will get to the voicemail menu.