Looks like you have a Cisco DPC3000 or 3010 HSI Modem, You would just replace that modem with the Cisco DPQ3212 eMTA Modem and use it for Phone and HSI. Cox will lease you the eMTA for free as long as you have the Phone service, it is an 8X4 Cable Modem.
The Phone jack is RJ11 and supports two lines. You would run this into the panel where your current AT&T line connects to the punch down block.
Cisco DPQ3212 eMTA Modem
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