phissed me right off. got a 2 yr contract via them to EE(orange) when the 2yrs are up i get £100. towards a new phone.
Bah, not now it seems. 
If the contract is with EE it should all be fine. Phones 4 u are just middle men after all.
Not true Im afraid, EE are Network Operators with Oftels permission to retail direct to the Public, Phones4U are Airtime Providers, not EE Resellers so the Terms of Contract are with Phones4U not EE.
EE will honour the contract, as the contract, and hand set are supplied by EE. Or Voda or whoever.
If you want the phone unlocked, you ring EE. More minutes, EE....;
Faulty hand set/insurance claim/loss or service/settings issue/text messages won't send, anything, you ring EE. As it's an EE contract.
Once the deal is done, contract signed, f4u wash their hands. Same with horehouse. Same with mobile phones direct etc etc.
The service provider then takes over, as the contract is with EE.
...although I'm not sure about the cash back bit...?
It depends on the actual contract. In some cases its with the network, with the shop being the retailer, but in some cases, Phones4U were the airtime supplier.
So it depends on the individuals contract here 
I wouldn't comment if it was Voda or any of the others, but orange/EE are very fussy about what software goes on their handsets as you know and what service plans are agreed. They simply don't let third parties in. Ime....
The point being is it isn't an EE/Voda/O2 phone, strictly, as per if you bought direct from the network. Its a P4U phone. Same as with the Carphone Whorehouse, or Dixons, or whatever its called this week.
Obviously dependent on contract, as all these places do also resell directly what the retail arm of these networks provide as well... ...which is why its not always straightforward who you are with.