It is possible that you can't use the AT&T phone with Straight Talk. If AT&T was your original carrier/service provider (per the tower mentioned) this could be part of your problem.
AT&T is a GSM carrier, while Straight Talk is CDMA in some areas and GSM in others. You need to first find out if the Straight Talk in your area is GSM or CDMA. Only Straight Talk can answer that one.
Now if the phone came from a different carrier/service provider (not AT&T) that will be a factor as well. Below are links to two tutorials on here showing lists of carriers/service providers.
CDMA Carriers in the USA ...
http
/www.tomsguide.com/faq/id-3143114/cdma-carriers-usa.html
GSM Carriers in the USA ...