First a note for the clueless: My definition of VOIP is a way to make phone calls using the cell phone data network or WiFi, and in certain cases those calls can be free. The methods I tested do not use a phone number, but instead use an email address or account name (although a phone number might be associated with the account in some cases). The best known example of free calling is Skype to Skype which is available on smart phones and computers, but there are many other options as well.
My main quest was to find a free and reliable way to talk a few different people using Android devices that connect to the internet via WiFi and/or a data 3G cell phone network. I tested some apps using two Android phones, and I would expect these apps to also work with Android tablets (for best results you might need a headset or earbuds with a mic, or perhaps a bluetooth headset). The Optimus Android phone did not have cell service, so it was tested with WiFi only (similar to an Android tablet without cell phone data service). Since many of the same or similar apps are available for iPhone and iPad, this might apply to Apple devices as well. Since my focus is on free calling, I am pretty much ignoring the many providers who are willing to provide VOIP minutes for a price (even though it might be a low price). Also I am focusing on how well some of these apps perform using 3G on the Android devices that I have access to. CNET has more complete reviews of these apps if you want to know more about them.
Now for the technical stuff: Every app I have tried works very well with WiFi at home, but I have had some troubles with high quality calling via 3G - probably due to lack of adequate and consistent bandwidth. See the section near the end for bandwidth usage of different apps. I have tried a few apps in search of something that would perform well whether connected via WiFi or 3G.
CSIP Simple via WiFi is just about the same as a wired IP phone or ATA (analog terminal adapter) except that it adds a fraction of a second extra delay. My limited experience with other WiFi includes hot spots where my phone would ring, but there would be no audio; or hotel WiFi where CSIP Simple was somewhat usable with sometimes broken audio. CSIP simple is the most bandwidth intensive app that I have tried, so if it works with a certain WiFi location, then all the other apps I tried work as well. Another well known SIP app is SipDroid, and there are several other SIP apps available.
CSIP Simple via 3G does not perform well for me. Usually calls drop after a few seconds, although I rarely have been able to maintain a call for several minutes. Perhaps one day I will figure out how to get the CSIP Simple app on my phone to use a lower bandwidth CODEC that Asterisk (PBX In A Flash) is happy with.
Tango is technically the best performer that I have found, although the user interface needs some work. Tango works fairly well via 3G and uses the least amount of bandwidth (for voice calls) of all the apps I have tried. The thing I do not like about Tango is that it is impossible to edit the Tango contact list. It seems that all of my contacts who have a Tango account show up in the list, and they cannot be removed from the Tango list. It is difficult to manually add a Tango contact. And I found myself accidentally dialing contacts thinking that a long press would bring up a context box allowing me to delete them.
Nimbuzz voice quality is a bit more likely to be broken than Tango in my tests, but it is not bad. Nimbuzz has a pretty good user interface, and it is easy to add contacts (even those without a phone number). You can manually add contacts one at a time, or import contact lists from various email and social networking sites that you may have an account with. Nimbuzz appears to be able to connect as a SIP device with any SIP VOIP provider similar to CSIP Simple and SipDroid, but I could not get that to work.
Skype to Skype is free and works with just about any smart phone or computer. Some people do not believe me because they think I am talking about Skype Mobile which is not free. I have found that even via 3G, Skype often has acceptable voice quality. Skype probably works fairly well via 3G because the bandwidth requirements are modest. Unfortunately Skype is unusable for me because I cannot hear the Skype ring tone when someone calls me, and the current version of Skype has no obvious way to change the ring tone or the volume.
Fring seems to be promising – it worked well via 3G during my limited testing, but Fring and the Motorola Droid X do not play well together. When the Droid X answers a Fring call, the microphone does not work. For some strange reason, when switching to a video call, the microphone turns on. However, this was a deal breaker for me, and I gave up on it. Another con: Fring is a memory hog using 15 MB of phone memory.
Tikl is not a VOIP app, but rather a walkie talkie app. However, there are some things I like about it. It has low bandwidth usage, and it can deal with high latency. I have not tried it via satellite internet, but I suspect that it would work fine.
With Tikl there can be several seconds of delay which would be a problem for rapid fire exchanges, but there is a redeeming quality that is not present with real 2 way radio conversations. When two people talk at the same time on a real 2 way radio, neither one hears what the other one is saying – they are both wasting their time and effort. On Tikl, the speech is buffered (stored) and played back after you stop talking. For those who feel compelled to keep talking forever, this would not work, but it would be useful for some people who can adapt to a 2 way radio style of communication. You can always use the ham radio convention of saying “over” when it is the other person's turn to talk.
VOICE BANDWIDTH USAGE
Results are based on the bandwidth graph available from my router running DD-WRT software.
CSIP Simple - 95 kbps. When voice detection is turned on, bandwidth usage drops to almost nothing when both parties are silent. When either party speaks, bandwidth is used both ways.
Tango - 35 kbps - bandwidth is only used for the person speaking, and drops when both people are silent.
Nimbuzz - 40 kbps constant (both ways)
Skype - 40 kbps constant (both ways)
Tikl - 40 kbps one way while speaking
DISCLAIMER: Cell phone data speeds and latency can vary substantially depending on location, signal strength, time of day, total data usage by other users, and probably other factors. Therefore, your results may be different than mine (your mileage may vary).
SUMMARY: Sometime in the future native cell phone voice calls may become just another data stream, and the cost per minute or per block of minutes will plummet. Until then it is fun to see what we can do to make that happen on a small individual scale. Now that you have an idea what to expect, maybe you will try out some of these or similar apps for yourself.