I have an Iphone 4S on Verizon and Siri works great. My GF has an Iphone 4S on AT&T and Siri seems to get only about 1/2 of the words correct when trying to send a text message. My GF can use my iPhone and Siri works great. So, it's not an accent issue. Are there any settings within Siri other than "Language" that will make a difference. Is the performance of Siri Carrier dependant?