A mod call button, tat a call is sent and ten you receive a report and like a admin gui can make?
You can do this multiple ways, you could do it through the Roblox Datastores, but that would require a Mod to actually be in the game.
A probably more preferable way you could do this is through the HTTP Service by connecting things like Twitter, or Discord via Bots to your Roblox game. There are actually a bunch of working Discord tutorials of how to do this if you search for them on the Devforums.
Unfortunately I believe Discord Webhooks no longer work due to violations of their rules on them in terms of limits.
It wouldn't be too hard to create something that sends a report to Discord to request a moderator as long as you understand remote events, sending requests to Discord and how to use a proxy. As we all know, Discord blocked requests from Roblox causing people to believe the above to be impossible however somebody on the DevForum has created a proxy allowing us to submit requests to Discord without breaking any of the Roblox or Discord rules (as long as you don't mention it submits to Discord to the players). I will link some useful DevForum posts on sending requests to Discord and related topics later on in this comment/post.
When coding a system such as a report system that sends a report to an off-site service (such as Discord) you will require HttpService to be enabled which will allow requests to be sent and received to and from the server. If this is not enabled your report system will not work as the request can not reach the external site.
Assuming your game is not in experimental mode (has FilteringEnabled enabled) you will also need to learn about RemoteEvents (used to communicate between the server and client). RemoteEvents work by sending a "signal" to the server that the server listens out for. This "signal" can carry data such as the reporter, description of the report and more.
Other information: Proxy: https://devforum.roblox.com/t/roblox-discord-webhook-proxy-server/98825 Roblox-Discord guide: https://devforum.roblox.com/t/discord-integration-a-guide-on-using-discord-through-roblox-updated/47090/62 Remote events: https://wiki.roblox.com/index.php?title=Remote_Functions_%26_Events
I hope this helps a lot! ~Emerald