o2 unlimited can or cannot ring giffgaff number for free or not

[#1]

as far as i know that giffgaff is connected with o2! i wanted to know that can my mates who have o2 unlimted can they ring me for free or will it charge them? 

[#2]

It will charge them or take it off them minutes allowance because 02 and giffgaff are two seperate companies/networks.

[#3]

Although GiffGaff is run using the o2 network, its also independant, so unfortunately o2 perks dont apply when calling GiffGaff, the same way GiffGaff to GiffGaff wont apply calling o2.

[#4]

Calls are not free as they're not calls to o2 numbers but giffgaff numbers

Giffgaff use the u2 network but count as a separate network, just like tesco mobile etc

[#5]

Unfortunatley it will cost them