Hi, how long has this been going for?
Which phone do you have?
Have you transferred a number to giffgaff from another network?
If so, when did that happen and from which network?
What does the person hear exactly when they call your number?
Call ##002# to switch off any diverts then call 1616 to switch voicemail back on
Also check that your do not disturb setting is switched off
Now call your own number from your own giffgaff sim
Do you arrive at giffgaff voicemail?
Text the word NUMBER to 2020 and also to 43430
Do you receive replies from those?