The original US female voice of Siri commenced recording in July 2005, and briefly mentions in this interview the improvements over time in the concatenation of text to speech.
YOUTUBE z2bTymnb1uE 2013 CNN interview with Susan Bennett, the original US voice of Siri
These improvements are primarily designed to address questions of human appeal, as detailed in this 2001 paper by Prudon & Alessandro link
The original voicing of Siri as female: gender as a service. Now users can change Siri to a male voice.
In Australia, text to speech services such as voicemail prompts can be changed from US accented English to British accented English, and more rarely to Australian accented English.
Here is an interview with the Australian female voice of Siri, noting that Siri cannot always provide Australian-relevant answers to users with Australian accents.
YOUTUBE VMbtOo02ALI 2012 interview with the original Australian voice of Siri, who is also "the GPS girl" and motivational corporate speaker. link