Second Life‘s users, who now average about 62,000 concurrent logins, now consume more than a billion minutes of voice communication per month. Amidst some controversy over the merits and demerits of voice in virtual worlds, Second Life implemented voice communication in August, 2007. Vivox provides Second Life’s voice communication using VoIP. Judging by its use, the service seems to be quite successful.
VoIP service per user in Second Life is huge compared to telephone use per user in the common earth-world. According to Linden Labs VP Joe [Linden] Miller, “52% residents have voice enabled all the time.” Rob Seaver, Vivox CEO, stated in the middle of last year that “50% of online [Second Life] Residents use voice on average; over 80% of those are in chat at any time.” That’s roughly consistent with current data, which indicates that voice-enabled logins engage in voice communications during 72% of their login time. Second Life users that enter Second Life in a given month average 12.7 hours of use per week. Hence Second Life users active in a given month spend about 9 hours per week in voice communication. For comparison, AT&T/Cingular mobile and fixed (home) telephone users spent only a total of 14.3 hours per month communicating on the phone.
VoIP service in Second Life is more like talking face-to-face than telephoning in the common earth-world. Voice communication in Second Life is 3D proximity-based, so voice-enabled users communicate by voice with voice-enabled users around them in Second Life. According to the Linden Labs VP, minutes are calculated when a user is “listening to an audio source or speaking, we’re not differentiating between listening and speaking.” As IYan pointed out, one person talking in a crowd of thirty voice-enabled Second Lifers seems to count for thirty minutes of voice communication. This isn’t surprising. If logins are talking in Second Life during 72% of their login time, that would be a lot of logins talking either to themselves or to other logins who are talking at the same time.
Second Life’s voice communication is interesting because it shows just how cheap voice communication is. A free Second-Life account allows unlimited, free voice communication within Second Life. Moreover, every Second-Life user with voice enabled is delivered a customized audio stream with audio input volumes dynamically adjusted for proximity. Hence, voice communication in Second Life is significantly more technically complex (and costly) than VoIP telephony. Nonetheless, voice communication within Second Life is free.
Voice communication has become a peculiar business. While voice communication costs are probably not much more than providing email, most persons are used to paying for voice communication. In the world of Internet services, the habit of paying is a rare and valuable service attribute. Second Life, not surprisingly, sees some “unique monetization opportunities” in its voice service.
Second Life plans to monetize its voice service in a way similar to SkypeIn online numbers. Second Life will offer AvaLine in Q3 2009 as a paid service that allows any land-line or mobile phone to call an extension representing a Second-Life avatar:
“AvaLine will be the first voice feature to be monetized after the Beta. AvaLine will be monetized by charging (in L$) for reserving an extension for your avatar. There is no per call fee, and callers will not be charged unless they need to make a long distance call into the access number. The L$ fee for this can be set to renew (or lapse) on a monthly or annual basis. The annual L$ fee will be 20-25% cheaper than buying 12 months.” [as reported on Massively] 
Skype charges for minutes of voice calls made from Skype users to non-Skype users (SkypeOut). Second Life’s road map shows “SMS Out” planned for (beta?) H2 2009. Second Life’s “SMS Out” will enable Second-Life residents to send SMS messages to any phone outside of Second Life.
One constraint on voice call directionality might be regulatory concerns. For example, under US 47 CFR 9.3, an “interconnected VoIP service”:
Permits users generally to receive calls that originate on the public switched telephone network and to terminate calls to the public switched telephone network
Not being an “interconnected VoIP service” lessens abstruse regulatory issues concerning other-worldly telephone calls.
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Here’s a spreadsheet with data and calculations concerning Second Life’s voice communication service.
 The measurement of average minutes per week implicitly (via the calculation of TMP) refers to players that played the title. I assume that they played the title in the month specified in the survey (March, 2009). Accurate specification of the universe is important. Second life statistics indicate big differences in participation across longer time frames:
Residents Logged-In During Last 7 Days: 568,328
Residents Logged-In During Last 14 Days: 722,501
Residents Logged-In During Last 30 Days: 998,851
Residents Logged-In During Last 60 Days: 1,401,863
A longer time frame increases the number of more casual users and lowers average minutes of use per week.
 Tatero Nino says, “we were told it was calculated based on *speakers* not listeners.” That’s inconsistent with the Linden VP statement, quoted above. Moreover, such a calculation would make no sense quantitatively.
 Skyp’e Online number SkypeIn page includes at the bottom right corner the question, “How much will it cost my friends to call my online number?” The link attached to this question is no longer relevant, and the question seems to have been deleted from Skype’s FAQs. But it’s a good question. Some other good questions: “How much will it cost my friends’ phone companies for them to call my SkypeIn number?”; “How much will it cost my friends’ phone companies for them to call my AvaLine number?” Intercarrier compensation is an important part of what makes telephone service different from email.