PUK

Two things:

1. If you're a postpaid customer, your PUK is available to see in your online account. See https://www.att.com/esupport/article.html#!/wireless/KM1011771 for details or if you're a prepaid customer
2. PUK is not needed at all to do what you want to do. Just move your SIM to the HTC Desire and it should work. The PUK is necessary if you have turned on SIM PIN prompt and entered incorrect PIN too many times. If your SIM works in the Alcatel phone but you're prompted for a PUK when you move it to Desire then something is definitely wrong, this should not happen