1. You cannot take any legal steps against the telephone company because generally the service provider do not furnish the call recordings to anyone who may request them.
They would furnish the same if available only to court or to police.
Hence you first ascertain that if at all she has the call recordings and it is a fact or false information.
Further, she cannot do anything with the call recordings or even if you really have an affair with any other woman outside your marriage.
It is not a criminal offence, hence she cannot take any lgal action against you even if you really are in a relationship with any other woman other than your spouse.
2. You just ignore it, you cannot take any legal action against anyone for this because no one will accept that they have helped her, moreover this is not going to harm you in any manner.
3. You can very well issue a legal notice but befor that ascertain if she is really possessing the so called call recordings, and have you heard it?
Without confirming the facts do not venture into any such legal acts, it may backfire and you will land up in more trouble.