VoIP has improved a lot since its first days, today a lot of multinational business are using it as a reliable way to keep stay in touch.
With more and more telecommuters, and business man working, and with cell phone roaming costs still high, VoIP is a real option for both big and small corporations.
And when you talk to a CFO about investing to save costs, you usually will see the checkered flag for your project.
Most of these projects will involve IP phones or ATAs, but you may use softphones too, which are very convenient for the traveler guy, here we will review some of the soft phones available for Linux.
Linphone is a mature piece of software, it uses SIP, for voice and video over IP, it can work as a stand alone application, and you can call other sip enabled devices just entering its ips on the dial window of Linphone, or you can configure it to use an Asterisk PBX.
There are binary packages for the most common distros, so try to install it using you package manager.
Ekiga, is the softphone for Gnome Desktop, it supports both SIP and H.323, also has support for webcams if you kernel recognize yours.
As with Linphone you may use Ekiga standalone, or with a softswitch, you can also create an account using the Ekiga configuration druid, but over all, it is a great soft phone to be used with Asterisk.
Here is a screencast to learn how to configure it, to work with Asterisk.
Twinkle is another great SIP softphone, which can be easy used with Asterisk, or any other SIP gateway, it is easy to configure, supports STUN server, NAT traversal, Instant messaging, and as the others, can be used in a phone to phone configuration, or using a Gateway / SIP registrant.
Check here to learn how to configure it:
Kiax is an IAX softphone, so it is intended only for use with Asterisk, its configuration is really simple, but keep in mind that you need to create the username in the iax.conf file, instead of the sip.conf file to be able to use this softphone, it is really a good option if you are working with Asterisk.
Skype is a closed softphone, it means you may not use it with any gateway, it uses a proprietary protocol, and only works with the skype Network skype site, but it lets you add contacts, call them by free, or chat with them. You may also call land lines and cell phones using a credit card or paypal to add credit to your account, the rates are really low compared with normal Telco rates, as there is versions for Windows and Mac, it is widely used, and maybe the most popular softphone, it also has the advantage that you do not need to configure your own PBX.
Gizmo is another network like Skype, but the good thing is that its softphone also support SIP configuration, and can be connected to an Asterisk PBX, while using its own network!, you may also configure is with your favorite IM network, MSN, Yahoo messenger, etc.
The configuration is really easy, and you add credit to call to outside numbers, using its network, or use you own PBX to make those calls, I really recommend the use of this software, which can be downloaded from gizmo.com you can also download versions for cell phones, if you are using Ubuntu it is in medibuntu repository, if are using Crunchbang Linux you just have to install it with:
sudo aptitude install gizmo5