Check with you state laws first. I know someone that lost their job for recording phone calls on his land line in his house. The feds got involved and he was let go. Definately say to the csr that you are also recording the conversation. Have them clearly pronounce their name what call center they are from and maybe an employee id of some sort. i would test the sound quality with a friend before hand though.
Try this program it looks promising.
http://1800message.com/products.aspx