If you see
(carrier grade services), there are still many issues to be considered. Before you sign up with a paid voip service provider and use it to teach EFL/ESL online, make sure that you try the different providers. Make several test test calls with the trial version, and compare it to another voip service provider.
VOIP Service Provider for free calls
Below is a list of paid voip service provider (Voice Over Internet Protocol) that you can use for teaching English online. Although VOIP can be engineered to be 99.99% reliable