Yes, when using Minutes or Data mode. MultiLine does support calls and texts over Wi-Fi, which is free and doesn't use the underlying phone plan. See Call Settings: Set Data or Minutes calling modes.
When you use Minutes mode, when you make and receive calls, MultiLine uses the minutes included in your cellular phone plan. For those with unlimited minutes plans, there is no added cost. For those with limited minutes plans, it is subject to company policy. Often the companies we work with will give a small stipend to cover the monthly charge.
When you use Data mode, when you make and receive calls, MultiLine uses the data included in your cellular phone plan. For those with an unlimited data plan, there is no added cost. For those with a limited data plan, it is subject to company policy. Often the companies we work with will give a small stipend to cover the monthly charge.
Note: The average data usage for MultiLine users is 100MB per month. You can see your usage by going to MultiLine Settings > Usage.