I have noticed the increasing number of 2nd hand shops selling phones (like Cash Converters) who think that T-Mobile locked phones will take Orange sims and vice versa.

They seem to think that because its the same company (same parent company, two different brands) and they can roam on each others network, that the locked phones can take both networks sim cards.

So... customer on Orange for example sees a phone with the EE or T-Mobile and Orange stickers that these shops use (CEX also do this) and thinks i will get that, will work with my sim. Buys it, takes it home and tries Orange sim only to find its locked to T-Mobile.

I had this with a Nokia 5800 Recently, bought as a temp to replace a lost phone. Bought it cheap from Cash Converters and wanted to use a PAYG Orange sim in it, only to find it had a T-Mobile lock. And i wasn't going to pay to unlock it so i took it back and explained to them it won't take Orange despite having T-Mobile and Orange stickers on it when it was on display. They tried my sim, "Sim Card Not Valid" checked it in case it had been blocked, it was clear.

They tried an Orange sim (one of their tester ones i presume) and that showed the same error message, so with that they said "oh for some bizarre reason, it is the same company but will only take T-Mobile sim cards" and gave me a full refund. These people are clueless!! So just beware when looking at 2nd hand Orange/T-Mobile (or EE) stickered phones in these shops

to be honest i hate mobile phone retail staff in general. Not because they're bad people but because they literally know nothing about the company they work for. Sure it's just a job for most of them but i really don't expect to walk into a t-mobile store and be told that the "iPad 3 will work on 4G straight away with out needing to upgrade to EE4G when it's launched"
(no it won't + frequencies are wrong for the UK)

Quite a lot of the staff don't know anything and just bs to the customers. I've seen it plenty of times

However i know not all are like this and i know two who are actually very good at their job and have all the knowledge they need.

I have a Orange San Francisco 2, bought from Orange as it is an Orange branded phone, and I can put a T-Mobile sim card in it no problem. I do with my T-Mobile mobile broadband sim card so I can use the Personal hotspot feature.

Could have something to do with the updates the phones get sent, the one that registers the EE name on the screen instead of Orange and T-Mobile.

Yeah i have seen Orange send sim updates requiring a restart of the phone, once done EE shows up as the network ID

Some phones are network locked to the network codes or Orange, Tmobile and EE, you can test it with an iPhone and also some Lumias - quite handy when you think your sim is broke and you find it works in the other network handset

All iPhones (if they have been plugged into iTunes and the newer Lumnias will take any of the EE Groups sims.

Not so sure of the older Nokias, dumb phones and Andriod based devices.