The best way I’ve found to make cheap phone calls, especially if you need to call abroad, is to have a broadband subscription and then use VOIP (Voice Over Internet Protocol). This means that you’ll make phone call using your Internet connection and in many case you won’t pay anything, it’s free. You’ll need a headset with microphone or one of the many existing telephones that can be be connected to your computer. There are also wireless phones that can be used with VOIP. You can even have only a broadband subscription a broadband connection without a landline, so you’ll save even more money.
There are several ways to use VOIP for cheap phone calls. One is skype, maybe you already know this. With skype you install a free software and then you can call all other skype users for free, anywhere in the world (from computer to computer). Also, you can send SMS and chat and if you and the other user install a videocamera, you can see each other. You can also call landline and mobile phones at much cheaper rates than landline and mobile phones rates. This is great especially from computer to computer and the connection quality is usually very good.
I personally need to call a lot of landline numbers abroad, therefore more than skype I use VOIP Stunt. You need to install a free software and then, as a new user, you are entitled to free calls to landline numbers of many countries (Europe, USA, Canada, some Asian countries) for 60 minutes. Then you need to register (just choose a username and password), deposit some money into your account and you’ll be able to call landline numbers again for free for 120 days (up to 300 minutes a week, which is quite a lot for personal use).
Tip: whatever amount you deposit, you always get 120 free days so, instead of depositing let's say €50 in one transaction, you should deposit €10 several times so you get multiples of €10 and multiples of 120 free days (pay attention to transaction fees, though!!!, bank transfer is the cheapest one).
For more comfort, you can even use VOIP stunt to make phone calls from your regular phone (not from your computer) at the same low rates (or free). Calls to other VOIP Stunt users are free (and you don’t use up your free minutes). You can use your credit for cheap calls to mobile phones and other countries, to send SMS. Chatting is free. What I’ve seen is that sometimes the connection quality is not very good, so I use both VOIP Stunt and skype.
There are also many other companies offering VOIP service, just do a quick serch if you want to see more of them