Industry observer Michelle Mastin recently ran a good thumbnail analysis of the most popular VoIP apps suitable for second lines -- as she says, since none of them allow 911 emergency calling, they shouldn’t be your primary line.

Toktumi's Line2 (News - Alert), a mobile communication app for Android and iOS devices, lets you make and receive voice calls and text messages over a Wi-Fi or cellular data connection. It’s easy to set up and use, but does require you to enter a forwarding phone number that will ring in case the app is offline. “I had no trouble calling out, but I ran into some problems trying to get incoming calls to ring on my Wi-Fi-only devices,” Mastin reports.

Pricing is inexpensive and easy to understand, and call quality is “better than most other VoIP apps I tried,” she said.

Tru App for mobile device call quality “didn't quite keep up in my tests,” Mastin said, calling the audio compressed and tinny, with “more latency than with a regular cell phone call.” She called it a good option if you frequently travel internationally, or if you want to have a local number in another country, otherwise, it’s expensive for the lower quality.

Skype (News - Alert), Has mobile apps for Android, BlackBerry, iPhone, iPad, and Symbian. Setup is “a bit more complicated due to the many options available.” Mastin said. Call quality was good, but “latency on the system was high, with a delay of several seconds between when a caller spoke and when the words reached me.” It has plenty of options for customizing a plan specifically to your business's needs, she reported, but “the many choices can feel overwhelming if you're just looking for a simple voice line.”

Google (News - Alert) Voice provides one phone number that you can forward to several lines based on who is calling and what time of day it is, Mastin noted, saying it’s available as a mobile app for Android, BlackBerry (News - Alert), and iOS; and “Sprint is now integrating Google Voice into all of its smartphones.” The mobile app is technically not VoIP since it goes through your phone connection, which costs you carrier minutes unless you add it as a favorite number.

Mastin found that Google Voice “offers a lot of useful services at no charge and can save you from needing a texting plan on your smartphone, but either you'll be tied to your computer for VoIP calls or you'll have to add another service to make VoIP calls from your smartphone.”

The bottom line? Google Voice is free, so it’s pretty hard to beat on price, but for a simple, easy-to-set-up alternative, “Line2 offers great call quality, an easy-to-use app, and a coherent price plan.”