Not everyone uses Skype, Slack, Snapchat, Whatsapp, Microsoft Teams - there are plenty of other chat and messaging systems, not least HipChat. And HipTalk Universal is an unofficial mobile client for HipChat that seems to have been regularly updated over the last few years - and documented to an extraordinary degree. It's available in both Windows Phone 8.1 and Windows 

From the Store description:

An unofficial mobile client for HipChat that helps you see which of your rooms have new messages.

Features:

• Open rooms and private chats. View old messages history, send new messages -
• Receive toast notifications for new messages in favorite rooms and private chats -
• Pin rooms and private chats to start screen as live tiles to see number of unread messages 

Not having an appropriate ID/account I couldn't really try this out, but I'm sure a few people reading this can weigh in with real world opinions in the comments?

Here's HipTalk Universal UWP in action, anyway (some promo shots):

Browsing HipChat rooms and contacts, and (right) a history of messages received...

The configuration options are impressive, these are just some of the options. If you're (or your company is) a HipChat user then this could be a very useful way to get involved from a Windows 10 phone or tablet.

You can grab HipTalk Universal for WP8.1 and W10M here in the Store. Apparently there's an In App Purchase at some point. Anyway, comments welcome if you have the contacts to try this out for real.

NB. For Desktop, there's a full Win32 client, of course. But this is perhaps the only way to get involved on Windows 'mobile'?