How can I get a copy of messages on one of my phone line on my account

You may have to contact your carrier for the SIM/eSIM that you use on your device. Apple Inc does not provide SMS (text) services.

Reach out to your mobile carrier's customer service. You can usually find their contact information on their website or on your monthly bill.

Clearly state that you're requesting copies of text messages for a specific phone line on your account. They might have different procedures and fees associated with obtaining records, so ask them for details. Keep in mind that there might be legal regulations regarding the retrieval of text message records. In some places, accessing someone else's messages without their permission might be illegal. Make sure you're aware of the legal implications before proceeding.