Can't able to connect the users above 25+

Hi Folks, I have developed an application for Education management can arrange their Virtual Classes, but we cant connect users above 20+, its shows that ‘Call cannot be connect because there are no calling accounts which supports calls of this type’ from the 25th user. I am attaching the screen shot for reference.

Does any one has idea about this issue. It would be helpful for us to release our app. I will expect your good reply.

Thanks
Harini