Try a video call then non VPN because I would think if this is a throttling issue, video calls may be affected even though I would think people don’t use that often on a phone on Skype so they could have just applied it on audio calls.
Have you raised this with skype as this could be a API config issue as well. They will see traffic to their api and check if some are being blocked/throttled network level or not.
Inferring from anecdotal reports of speed issues it would appear giffgaff get a percentage of the available bandiwdth on a cell so a speed test will vary throughout the day depending on the load on the cell.
The ping looks ok (for VoIP latency of up to 100 ms is usually tolerable) but I would carry out the test again use a manually selected server on the UK mainland as traffic traversing submarine cables can sometimes add a bit to the measured latency.
Skype, Youtube and the like tend to use as much bandwidth as they can obtain to improve quality so has to be managed to avoid heavy load on the network, I don't know if O2 and giffgaff do block it but it maybe that they block the voice function to avoid heavy bandwidth use and rapid use of any allowance which it will do if it can, obviously when traffic is encapsulated via a VPN the network sees nothing but an SSL or TLS session
I think the networks that do support it have some sort of internal server that can control it's bandwidth use.
Perhaps for video but not for a purely audio call.
For a purely audio call, Skype gobbling all it wants to, you're looking at somewhere in the region of 40-60MB/hr on the data consumption front.
There is no network in the entire United Kingdom who are going to be troubled by or concerned with that low level of usage, even if ten thousand people are doing it all the time, (high quality internet radio will push the same data volume).
What I'm wondering is if Skype still using P2P how it used to and whether Telefonica are identifying that as file sharing and deliberately slowing it and dropping packets, but I'd need an active connection to test.
That seems much more likely to me.
I'll try getting out later to get a voucher and see if I can diagnose.
EDIT: Just tested on a non-telefonica connection running at 115/32mbps and it used less than 20KB/s throughout the call, so I was a little off. It's 72mb/hr at full pelt, but still well below what should and would trouble any ISP mobile or otherwise, in the United Kingdom.
Could be that yes, but Skype now is client to server as opposed to P2P but probably uses the same ports so could still be tripped up by traffic management based on ports.
I found Skype to be crap years ago so haven't dabbled since.
Found this in case it helps
If you wish to raise a complaint have a look here https://giffgaff.com/support/complaints
As @k89bpa has said it might be more as a result of Skype historically being P2P (as I said it it's not an app I use)
@glaswegianeric is it just Skype that the issue happens with or does it affect all forms of VoIP do you know?
@k89bpa just wanted to add both myself and my wife have a 2nd phone with international sims to call Pakistan
But we mostly use viber and whatsapp on our giffgaff sims to call Pakistan whether on wifi or mobile data works fine without having to change any settings
I have skype but never tried it.