BYOP SIM choice

[Rebel60]

I went to the ST website & clicked on the buy sim now. It gives the following 4 choices:
Phone is a Blackberry, or branded TracFone, SafeLink, NET10 or Straight Talk
Unlocked Phone (Phone must operate in 850 and 1900 MHz bands)
AT&T Compatible Phone
T-Mobile Compatible Phone
I chose the unlocked phone & entered my choice of sim (I actually tried both to see the results) and entered my zip code 39507, both SIM's are for AT&T. I don't get to choose T-Mobile.
The only way I can choose T-Mobile is to buy a T-Mobile branded phone, select that & then I can get the T-Mobile SIM.
I thought that as long as the phone was "Unlocked" I could choose which SIM I preferred to use. This appears to not be the case. I have to buy a T-Mobile phone in order to use T-Mobile. So way even list the "unlocked phone" option? You don't have a choice.
Yes, T-Mobile is available in my area.

[John]

Hi Rebel60. My name is Straight Talk John and I will be addressing your question. We have checked already in this. In order for you to get the T-Mobile SIM card, you may select the T-Mobile compatible option and proceed to purchase it. It will work with your unlocked GSM phone. Thank you.

[chrisbean2]

Do you HAVE to specify if its a att or t mobile phone?
If its unlocked wont it work no matter what?