Popular voice over IP provider Skype experienced an outage on Thursday, possibly leaving millions of subscribers without phone service.

Subscribers have been reporting issues with Skype, saying they're unable to log into their accounts and receive phone service.

Skype, which has over 220 million subscribers worldwide, issued a statement that links the outage to a software problem. "We expect this to be resolved within 12 to 24 hours," the provider wrote on its website.

Subscribers are encouraged to keep their Skype clients running, so they can be logged in as soon as the issue is resolved.

Downloads of Skype also have been temporarily disabled.

Skype is not the first provider to run into problems with VoIP. Earlier this month, another VoIP provider TeleBlend experienced an outage that affected about 2% of its subscribers. Some didn't have service for over a week.

Last year, Comcast experienced outages with its broadband and VoIP services in the Northeast region of the U.S. Subscribers complained about losing phone service for several hours. Also last year, U.K. telecom provider BT Group reported that its Broadband Voice service crashed, affecting thousands of customers. In March, Vonage customers posted complaints to an online forum, citing problems of making and receiving phone calls from their Vonage phones.

Issues that can cause disruptions in VoIP services vary. In general, VoIP is considered to be more unreliable than the Public Switched Telephone Network since it relies on data connections to transmit voice. Subscribers take a chance when they sign up for VoIP services, but it's up to the providers to address issues as quickly as possible or ideally to eliminate them altogether.