iphone will not unlock to any network

Hi Folks

Can anyone help please I'm hitting a brick wall here?!

I recently ended my contract with Three and asked them to unlock my 2012, 16GB iphone to any network.

Three are adamant they have now done this - but I can still only get service with a Three sim. When I first bought the iphone in 2012 brand new it was IMEI blocked and Three had to unblock it - they claim it is now still fine (despite appearing blocked on checkMEND website).

I have followed the itunes restore instructions to the letter, but I am not getting my phone unlocked. (itunes version 12.1.2 on imac, ios 8.3 on iphone)

I have tried o2, vodafone and EE sim cards - none of them get service (well actually the EE one does for 10 minutes then reverts to no service) - all these sims work ok in other phones.

I have tried toggling airplane mode, changing carrier signals, and updated carrier signals - nothing. Signal in my area is ok for all networks

Any help greatly appreciated

Thanks

Barney