From: Rabbi Moshe Sofer
To: Unknown recipient
A man who admitted paternity before ambush witnesses may retract his out-of-court admission without providing a pretext, since he was unaware of the witnesses and his statements constitute idle talk made before any support obligation arose. Following community practice based on the Tashbetz, the man need only swear he does not know the child is his and is then exempt from support. Additionally, voice recognition alone is insufficient to extract money even with two witnesses, and certainly where one witness relied solely on such inherently estimative identification.
Source: Shu"t Chasam Sofer CM §2