Phone can hear caller but they cannot hear me (Only single contact)

I have a strange problem where If I ring a person then I can hear them fine but they cannot hear me. However this only applies to this specific number as calling other phones works fine. This problem gets even stranger due to the fact when trying to test for causes the problem actually ended up reversing so they could hear me but I could not hear them, although this only happened once. Finally to add to the strangeness the problem on two separate tests resolved itself only to then return during the next call.

When I speak to the other caller if I speak quick enough they will hear me for perhaps a fraction of a second before they can no longer hear me.

Both phones are not in silent mode, neither have blocked numbers or call forwarding and both phones are capable of receiving calls just fine from other numbers

It’s only when these two phones try to ring the other does it happen

Phone 1 is an I-Phone 8 on the Gifgaff network

Phone 2 is a Samsung S7 Edge on the PlusNet network