When they move over to EE (around the end of November),prices will go down to 8p a call and 4p for a text.
See:
http://www.asdamobile.com/Currently they are 10p a call and 6p for a text.
A £5 bundle will also be available 100 minutes ,2000 texts, and 100 mb.
Looks like they are trying to match Sainsburys new payg standard prices