Hi
Since the conversation is in person and you have used the phone as a recording media,it is perfectly accepted evidence under section 65B of indian evidence act.
The next steps are
a) Keep a copy of the recording in CD by transferring the content from Phone to a CD
Many people attempt to transfer the contents directly to CD and in the process end up in committing mistakes and end up with the courts dismissing the evidence in CD format.
So it is highly recommended that you approach the any of the following forensic labs for transferring the contents of the audio recording from Phone to CD .
a) Truth Labs, Flat # 19, Dev Apartments, 1st Main Road, Kasturba Nagar, Adyar, Chennai - 600 020.(www.truthlabs.org)
OR
b) Forensic Science Department/Forensic Laboratory, 30 A Forensic House, Kamarajar Salai, Mylapore, Chennai - 600004
These forensic labs will help you download the content safely and securely in accordance to law and they will also issue certificate on request with date and time stamp of recording on it .
Both of these above forensic labs are approved by Government of India and Tamil nadu and their certificate is valid in all courts including the supreme court.
These institutions will charge anywhere between Rs1000 to Rs1200 for their services but will ensure a safe, secure and authenticated data transfer.
Hope this information is useful.