You can't use the ATT no contract phone on Straight Talk. The unlocked version is $399. Still a great deal.
I just picked up a 950 at the Microsoft store (because ATT online store was out of stock on black phones and no local ATT stores had any at all) and at the ATT price. I thought it would be as easy as the swap from my 920 to my 640, but the 950 uses a nano(?) SIM rather than the micro SIM. No problem. I just called 611, told tech my cell number, the IMEI of the new phone and the ICCID for the new SIM. All was switched over in under 5 minutes.
Don't forget to make sure Dad has a current backup!