Hi @james17667,
Have you tried a different set of known working headphones in the phone, to prove that the phone's headphone jack is OK? Sometimes minute differences in the sizing of the headphones' plug connector can mean that it doesn't operate the contact inside the headphone socket. Try gently moving the headphone plug in and out and a bit side to side to check if the phone now detects it. Be gentle when doing this
What do you mean by "new ..... headphone jack"? Is this because it is a new phone or is it already a replacement headphone jack in the new phone?
What is the make and model number of the phone?