It scales to a couple of thousand users, so I don't see any challenges there.

Around 15,000 users are using it within your company

Scalability is not my concern. I'm just a user of the application. I only use it for meetings, and I don't have to see whether it is scalable or not. I'm not using the application from the delivery perspective.

Scalability is good. 

We began with a small number of users and it has increased. We found that it was easy to scale.