To identify a phone number's line type with Twilio Lookup and Node.js, developers can use the Twilio Messaging API to query the Twilio Lookup API for information about a phone number. The lookup API provides carrier information, which can be used to determine the line type of the phone number. The possible values for line type are mobile, landline, or voip, and this information can be obtained using the Twilio Node library in JavaScript. By running the provided code example, developers can see what comes back when querying a phone number's line type, which can be useful for businesses that require specific line types to sign up for services such as VoIP numbers.