If you want it to make it in to court you have to have the consent of the other party.
Seriously, dolly99, get a lawyer and ask them these questions. We don't even know if you're a fit parent.
you must by law nothig that person or inadmissale. its illegal to do in the us.
On a mobile yes just press record. On a land line yes if you have an answer phone just press record.
The only way you could use such a recording in court is if the party knows they are being recorded. A voice mail would also be admissible, as anybody leaving a voice mail knows they are leaving an electronic copy of their message.