Proxima's employees also use peer-to-peer calling to avoid toll charges when traveling. Users make calls via their laptops rather than incurring long- distance or mobile roaming charges, especially during trips abroad, says Ehr.

Proxima's CEO recently purchased a dual-mode PDA phone so he could use Skype's Pocket PC client over Wi-Fi -- and uncovered a potential problem with peer-to-peer calling. The PDA lacks the power required to make Skype calls. "If you are the originator [of a call], your machine is doing all of the processing," Ehr says, and conference calls increase the workload even more.

Skype and similar programs also lack centralized management capabilities, such as the ability to review and retain call detail records, and they may represent security risks, says Lazar. "For companies subject to Sarbanes-Oxley or HIPAA, that has been the showstopper," he says. For other organizations, however, the choice is less clear.

Marvin Wheeler, chief operations officer at Terremark Worldwide, a collocation services provider in Miami, says he sees remote users increasingly calling in over services such as Google Talk or Skype. "For spot use, it's great," he says.

Peer-to-peer voice services are still consumer- focused and offer few features to support business needs. Most lack a well-designed, central directory, so each user must maintain his own list of user IDs. Skype users must set up a prepaid account to cover per-minute SkypeOut charges or monthly fees for a SkypeIn number. Skype does allow administrators to set up a common pool that specified employee accounts can draw against, but invoicing and detailed call billing aren't available, and individual user IDs must be configured and administered individually. "I need an account. I want to be invoiced," Ehr says.