Vonage has released an iPhone app that offers some benefits over Skype's competitive voice-over-IP (VoIP) calling service.

The new app, Time to Call, is available for free now in 87 iTunes stores around the world. Users will be able to make 15-minute calls to people in 100 countries for around £1.50. That rate applies for calls made to both landlines and mobile phones.

The service works on Wi-Fi worldwide, and in the U.S. and Canada callers can use Wi-Fi or 3G. When on 3G service, callers may pay more, depending on their service plan with their operator.

Even with more expensive calls to landline phones, Vonage might be attractive to some people for its convenience. Users pay for the calls on their iTunes accounts. They don't have to set up a subscription or use Vonage for their home phone service.

Skype requires people to either pay for a monthly subscription, which includes cheaper per-minute rates, or fill a credit account with Skype before making calls.

Both services are attractive to people who travel internationally. They both offer significantly reduced calling costs compared to roaming with a mobile phone. Vonage is offering one free 15-minute call to people who download the app.