With the rollout of RemoteSpark version 1.9.91 we're receiving reports of users having trouble making calls. The most common reason behind this is a version mismatch between the devices.
The simplest way to resolve the issue is to ensure that you've updated the software on both devices (HoloLens and PC) to the latest version of RemoteSpark.
If you're having trouble confirming which version of the software you're using please consult this article.
In the majority of instances, this will resolve the connection issue.
If you're using InstantAssist and thus the RemoteSpark web client and experiencing this issue it's likely that you're referencing an older version of the web client meant for an older version of the RemoteSpark app. Please ensure that you've cleared your bookmarks and have followed the link generated from the InstantAssist call sent from the application.