The 3120 has no FXO or FXS ports, so presumably you were using IP phones or an ATA (analog telephone adapter) to get POTS dial tone. As a rule, cable providers supply their own equipment with regular RJ-11 jacks for telephone service over cable.
You could continue to use the 3120 as a NAT-router/firewall/4-port switch behind the cable company's gear, but it likely won't be involved in the voice part of the network. To be sure, ask the cable company what they're providing.