Skype is generally used as a software application running on a computer equipped with a microphone and speakers or a headset. But subscribers will also have the option to call a local number from their phones and be connected to international numbers that fall under their plan, paying only local access charges or using their cell-phone airtime.

Unlimited international calling plans have been popping up in recent years from hardware-based phone services like Vonage International Holdings Corp. and cable companies, but the prices are generally higher, and the plans are add-ons to basic calling plans that cost even more.

Skype said its subscribers called phones for 1.7 billion minutes in the first three months of the year, compared with 14.2 billion minutes used in computer-to-computer sessions, which are free.