If your nokia is a new phone, you just need to take the sim card out of your old phone and put it in your nokia.
Assuming that your nokia is either "unlocked", or locked to your current network, then it shouldn't be a problem .... if not, then you'll have to get your nokia unlocked.
Do the phones accept the same size sim card??
What make and model is your current phone??
What happened when you tried putting the sim in your new phone - did you get an error message of some kind??
If your 'new'phone is around 15 years old.. how old is your old phone??
It would be throwing good money after bad to start unlocking a 15 year old phone. For not much more than the price of unlocking, you could buy a new phone.
I'm wondering if the OP is referring to their current phone, when they mention a Nokia 6500 slide.
I didn't realise it was a dinosaur until you just pointed it out.
According to the following link it was first released in 2007: