[edited on 2014-11-07 to mention cordless phones that work with mobile phones]
Making voice calls using the telephone network is now antiquated. There are practically no good financial reasons to have a landline anymore if you can afford Internet at home and a smartphone. Consider this blog post an open letter to my parents and in-laws where I attempt to convince them that having their landlines is not necessary and that there are better, cheaper options to talk to me and everyone else who isn't a free phone call on their mobile phone already.
In this post I'm working from the assumption that everyone has at least one smartphone with a non-data phone plan in the house and everyone has wireless internet throughout their home. I'm also assuming their phone is used to make calls and receive calls and nothing else, e.g. no fax machine. I'm also going to use pricing and such from Canada because that's what I'm most familiar with, but since we have some of the highest telecom rates in the world it means I'm being somewhat liberal for my US family members when making price comparisons (but as you will see, essentially anything above free is wasted money when it comes to a landline).
When making a call from a landline you need to consider local, long distance, and international calls. Let's assume you're living in Vancouver, putting you under the purview of Telus. Local calls are unlimited and free with your monthly charge of $30/month (past first six months, no bundling discount; otherwise it's $25/month in a bundle). You're going to call people outside of your local calling area which means long distance, so we will toss in the $6/month for 300 minutes of Canada/US calling ($0.02/minute if you manage to use all the minutes). Internationally it's e.g. $0.05/minute to call the UK. So that makes the cost range from $25/month if you have another Telus service and never call outside your local area to let's say $38/month for no bundling discount, 300 long-distance minutes, and a 60 minute call to the UK.
Now let's look at what you can do with your mobile phone. From the outset I said that I assumed people had a smartphone with the most basic plan available; that's the WIND 25 plan from WIND Mobile. That gives you unlimited Canada-wide calling but no data. So right there for $25/month you cover everything but any US calls as part of the $6/month Canada+US add-on for a landline and the 60 minute call to the UK. Now WIND charges $0.15/minute for any Canada/US calls and $0.75/minute to UK numbers (landline or mobile).
But really none of these extra costs matter. If you install Google+ Hangouts you can make any of these calls cheaper than a landline or through WIND using nothing more than the WiFi in your house and your Google+ account. And I really want to get the point across that since this uses the WiFi in the house you are not gaining or losing anything compared to a landline in terms of accessibility. This means lacking a data plan is not a problem, nor is lacking good cell service in the house since what I'm about to suggest is over the home internet connection.
If you're on an iPhone or iPad you need Hangouts for iOS and if you're on Android you need both Hangouts and the Hangouts Dialer. With that software installed you can use Hangouts to make phone calls to any phone number in the world. All calls to the Canada and the US are free, with calls to the UK being $0.01 - $0.03/minute depending if you're calling a landline or mobile. That makes Hangouts either equivalent or cheaper than either the landline or WIND for Canada or US calls depending on distance, and the rate to the UK is less than either a landline or mobile phone call no matter what.
Now there is one drawback to making calls this way and that's the caller ID will show up as "Anonymous" for the person you are calling. If you live in the US, though, you can get Google Voice and that will make the call show up as your Google Voice number. And since you can have Google Voice ring your landline and in Hangouts on you mobile phone simultaneously you can help transition people over to that number before you cancel your landline and have people not notice the switch (or you can port your landline number to Google Voice and not have to tell anyone you changed numbers). And if the people you are calling happen to have Hangouts installed then you can make a Hangouts-to-Hangouts call which will show up as you. The call will also be much clearer since the internet can send better audio than the phone network can.
Short of not being able to make a call while the power is out (although you can always make a call to 911 regardless of whether you pay for a landline), you don't lose anything substantial from dropping your landline. You can even keep having cordless phones! Companies like VTech have options to let your mobile phone connect to the cordless phone base using Bluetooth. And at least in VTech's case you can have two mobile phones actively connected at once, preventing you from having to decide whose mobile phone is going to be connected to the cordless phone(s).
Away from home
Now mobile phones are mobile, so what about when you are out and about? Can the benefit of using Hangouts for phone calls be extended to when you are away from home? The answer is yes.
When I have used Hangouts to call a landline I have noticed the bandwidth used is roughly 750KB/minute. WIND charges $0.05/MB when roaming. That means making a call while roaming using Hangouts less than $0.04/minute. That is less than a call to the UK on the landline. It's also cheaper than a roaming call on WIND. So even in a pinch it's still relatively cheap.
But let's say you bumped up to the WIND 35 plan that gives you unlimited provincial calls but also unlimited data. In the end it really isn't going to matter that your free calls go from national to provincial since you will use Hangouts for any non-free calls anyway. So for an extra $10/month you can now make the same free calls anywhere you happen to be with your mobile phone along with mobile data to check your emails, etc. And if you compare the worst-case costs for the landline, it's $37/month vs. $38/month for the 300 minutes of Canada + US calls and an hour call to the UK, or $35/month vs. $36/month if you leave out the UK call. The only way to shave $4 off your monthly bill compared to a mobile phone is to have a bundle with Telus ($25/month + $6 for 300 long-distance minutes).
But this is assuming it's mobile vs. landline. In fact it's usually mobile and landline, which makes this comparison borderline pointless and simply shows there is literally no reason financially to keep a landline anymore when looking at it from the perspective of making a phone call (and if anyone says "911" then I will point out that by law, phone companies have to service a 911 call even if you don't pay for a phone line so you can keep a phone plugged in for emergencies if you want).
If you ditch your landline, how do you receive calls? Well, presumably people have your cell number so you can just answer your mobile phone. As mentioned above, if you have multiple phones in your house this can be the single inconvenience you lose at the price of saving some money every month by ditching your landline. Now if you someone is calling your Google Voice number you actually mitigate this inconvenience as Hangouts will ring on your mobile phone thanks to the WiFi in the house, your tablets, and any computer you're logged into where you have the Chrome extension or Chrome app installed or are logged into Inbox or Gmail (the app is Chrome OS and WIndows only, the extension is any OS).
The only other inconvenience from ditching the landline at this point is what to do when the power goes out. That requires actually examining how often you make or receive phone calls during a power outage in a year compared to a couple hundred dollars (but do realize that's enough to buy a new Chromebook every year so it's not a small amount of money).
Now my entire family has Google+ accounts, so pointing them at Hangouts is no issue. But if for some reason you are not in such a position, you do have some alternatives for making voice calls. You can use something like
Facebook Messenger or Skype, but the former only supports Messenger-to-Messenger and Skype charges to call a phone number. Talky lets you do video calls using nothing but your browser as long as it's Chrome, Firefox, or Opera (which I hope it is for anyone's sake in providing tech support from a security perspective). Lastly you can always switch to a VoIP-based solution like Shaw, Vonage, or Ooma if you truly cannot let go of your landline, but that's still throwing money away compared to just using your smartphone and Hangouts.
Using a mobile phone and Hangouts to make calls will cost you caller ID, unless you live in the States in which case it will simply come from your Google Voice number. It will also require you to use a device with Hangouts installed on it to place a call (mobile phone, tablet, or laptop). For receiving calls you only use your mobile phone, unless you have Google Voice in which case any device you can place a call with will also be able to receive a call to your Google Voice number (complete with transcribed voicemail).
For all of those "drawbacks", you get to drop your landline and save a couple hundred dollars a year. You also simplify what is required for people to call you by having one less phone number for them to try calling. And by actively using Hangouts to make calls it means you will also be actively using Hangouts itself, which means you can also use it for messaging. Anyone with children will enjoy that as using Hangouts means a much easier, more prompt reply to communication compared to trying to get a hold of your children with a phone call. And with Google Voice you can even send SMS messages if your children are "old school" and have not transitioned over to messaging apps yet. And finally, it means when you travel there won't be any issues placing or receiving calls since almost everyone has WiFi where they stay.