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I am thinking of buying a Galaxy Ace mobile the guy in Tescoa said the they could do me a good deal but it would mean that i would have to put in a sim card for a Blackberry My question is this :- is the blackberry sim card compatable with the Samsung phone or will I get problems?

Sims are issued by the network providers, so you have an 'Orange Sim' or a 'Vodafone Sim'

There is no such thing as a 'Blackberry Sim'
Phones are normally locked to 1 network, so if you but an Orange branded phone, you have to use an orange sim unless you get the phone unlocked.
They do this so they can discount the phones.

If you buy an unlocked phone, normally called 'Sim Free' then you can put any sim in but you will have to pay more for the phone as it will not be discounted.

Without more details I can't say what the guy is trying to sell you, but the one thing you can be sure of is he is thinking more about his commission than giving you a good deal.