He uses the old phone, cabled to the DSL modem's ports is most likely true about the phone cabling and phone used with his new DSL installation.
b. He uses the old phone, cabled to the DSL modem's ports.
Explanation:
Since Fred want to connect old phone if it has Wi-Fi connection and if DSL modem as Wi-Fi enabled or exists it is easier to connect. If DSL modem has only available Ethernet port end user has purchase data cable convertor which suitable to the Ethernet port and tries to connect old mobile phones.
Getting data cable with Ethernet card convertor is mostly not available for old mobile phones. News day all DSL modem has Wi-Fi enabled features. Either end user has purchase DSL modern with Wi-Fi.