Device locked

I went to promenade mall yesterday and I pay my account after I requested a phone I bought with the account,fine the lady assist me she told me that the phone is a dual sim since is locked for MTN only but I can use other network for sim 2 "she said" I put my primary sim which is the current number I'm using (Vodacom)and it require a pin and SIM ME pin (np) I have no idea what is that,today I went there some guy help me and couldn't help anyway instant he told me to go to MTN shop I ask how but I thought it here to them he said sorry and give up i ask him why not them log a request for me to MTN or call them to solve this because I need to use the phone with my old sim card on I can not change my number and the network Im using yes I don't have problem using them both but at least my old number and sim must be on a phone.so I left the shop with heart broken that time I will pay the phone that I'm not even using.😢😢😢😢😢😢😢