Has porting not properly worked?

I ported my old number to Smarty a couple of months ago and am using the Smarty sim in a new phone. I purchased a new (not smarty) sim for my old phone which has a completely new to me number. Both phones are iPhones and are using my Apple ID. Occasionally, I am still receiving calls to my old phone when the caller has dialled the number that is attached to my new phone! I am baffled! How can that happen? The sim that is in my old phone has never been associated with the number on the sim in my new phone. Does anyone know how I can prevent this from happening?

Thanks