Alternatives to a Second Phone Line

T-Mobile's HotSpot @t Home
If your issue with using your cell phone as a second phone is the fear of racking up too many minutes, T-Mobile's HotSpot @Home service might work for you. It works with certain cell phones that are Wi-Fi enabled. When you're out, you use it as a regular cell phone, but when you're home, or within range of any T-Mobile hotspot, your calls automatically switch to Wi-Fi, saving you cell phone plan minutes.
VoIP Phone Service
Many VoIP phone services charge less than the phone company charges for a second phone. Installation is also either free or low cost, and you can usually do it yourself. No wiring is required beyond plugging in another device between your broadband modem and your computer. Vonage is the best-known service that allows you to make regular calls to anyone with any kind of phone number, and to use your regular phone. Other services exist, and your cable TV provider may have a similar service. Monthly fees of between $20 and $35 apply, as of 2009, giving you unlimited local and long distance calling and low-cost calls to many other countries as well.
VoIP via Computer
Services such as Skype are often free if you call only people who are also members of the same service. Typically, you need to sign up for a user account and download software, and your computer needs a microphone and speaker, or you need a headset with a built-in microphone. Some of these services let you pay a small fee, usually a yearly fee of $10 to $30, to purchase an account with the ability to make and receive calls from people with regular phone numbers, but you usually still have to do it using your computer, or purchase a special phone device.
Google Talk and Other Instant Messaging Apps
Online chatting or instant messaging is a way to communicate in real time without a phone. But many providers of IM, including GoogleTalk, have a voice option. It works similarly to the way Skype works, letting you talk to another member of the same service using your computer hardware and the Internet connection. These services are usually free.
Google Voice
Google Voice is not a second phone line but a way to manage your various phone numbers. Sign up for Google Voice and obtain a phone number. Give that number to your friends and family instead of your other numbers, and you can configure how it works. Specify that when family members call, it rings at all of your numbers. But when those annoying telemarketers call, it will go straight to voicemail. Change which phone it forwards to depending on your circumstances. Check voicemail from anywhere on the Internet, and forward messages to others. Google Voice is currently available by invitation only, as of mid-year 2009.
Internet Call Waiting
If you have dial-up Internet and you don't want to miss calls that come in while you're connected to the Web, you can get a service such as Phone Tray, which tells you who is calling and gives you the opportunity to answer the call. PhoneTray has a free version, or you can pay a one-time fee of less than $20 for the full version as of 2009.