We have many users who successfully call into our conference lines using VoIP services such as Skype, Vonage or any other telephone company that provides services over the internet. However, VoIP services can be more problematic and less consistent than using a land-line phone.
VoIP lines utilize the internet to transfer audio, and because the Internet can have varying speeds as it moves from station to station, these lines can have inconsistent levels of voice quality.
Also, if there is congestion on their side, it's possible that you'll have difficulty connecting to some numbers or even get disconnected during a call, much like a mobile that is in an area with a poor signal.
We suggest avoiding calling in with a VoIP service during peak business hours, such as during the morning or late evening.