I'm not familiar with giffgaff, but a little searching around the internet makes me think it uses the O2 network, so pay as you go phones that are branded to the O2 network would work on giffgaff. If that's the case with your phone, you'd need to complain
to O2.
I don't have much else to contribute to this thread and so I'll leave it up to you to perform any additional research you need to satisfy your own curiosity or continue down this path. Good luck to you.
Okay thanks for your efforts @profjulie, though I'm still bemused as to how giffgaff/O2 managed to remove these features when I put the SIM into my new phone, which I'd purchased completely independently. I guess I must be missing something basic here. I'll
get in touch with giffgaff but I don't think I'll be suprised by their reply.