Well, I have read critisisms about members not looking through existing posts before putting fingers to keyboard and posting a duplicate comment. So I looked for an existing post first. Is network locking no longer an issue? If so, some cellphone shops are still misinformed, since they maintain that the phones they get are network locked. I called Vodacom while I was at the shop but after 20 mins on hold, gave up.
A short while I spoke to Vodacom who said that while phones should not be locked, the procedure for inserting another provider's SIM might not just be a matter of taking out the Vodacom SIM and replacing with a CellC or MTN SIM. You must first go to the networks menu and ensure that other networks are available, then only insert the SIM. It has to do with how manufacturers interpret the instructions for the firmware.
Of course this might be a lot of hot air, and can even be a faulty handset, but when you insert a foreign SIM into a phone and the msg "Invalid SIM" appears, you need to know how to deal with it, and the excuse given by the salesperson might be the one that sprang to his mind.
Certainly in the UK, phones are locked. If you go to Carphone Warehouse, the identical prepaid phone will have 2 prices. A cheaper one for Vodafone and another for SIM free as they describe it.