When you join giffgaff, your voicemail is automatically switched on. You can access it by dialling 443. You can switch it off by dialling 1626 and reactivate it by dialling 1616.
To reset your voicemail PIN text RESET to 802443. You'll then receive a text with a temporary 5-digit PIN. Call the voicemail from your giffgaff phone to then enter a new PIN.
Calls to voicemail are charged a per call rate of 1 minute out of your goodybag allowance / airtime credits. Calls to the voicemail from abroad are charged as a 1 minute call at the applicable rate for this country.
iPhone visual voicemail
Visual voicemail is not available on giffgaff but applications can provide a good alternative. More information on how to set up the voicemail on your iPhone is available here.