I think you will have trouble getting a notification specifically to WhatsApp because it's a proprietary app without an official public API (from what I can tell). The top relevant results when looking for WhatsApp notifications just generally were more proprietary services mostly targeted at businesses. Not a promising sign. I found some claims of open source whatsapp apis, mostly JavaScript/browser based but I have no idea if they work (reliably) or how easy it would be to integrate with Jellyfin. I could be wrong, but I didn't find many promising results after about 10-15 minutes and as a software engineer I basically google tech shit for a living.
Your best bet is one of the options spelled out with the Jellyfin webhook plugin or Jellyseer (which looks like basically the same options at a glance):
- https://github.com/jellyfin/jellyfin-plugin-webhook/tree/master/Jellyfin.Plugin.Webhook/Templates
- https://github.com/Fallenbagel/jellyseerr/tree/develop/docs/using-overseerr/notifications
Some of those would allow your users to get notifications by installing a special client on their phone. If you want to selfhost try Ntfy, if you don't try Pushbullet ($5 OTP). so for example you can run a Ntfy service on the same machine as Jellyfin, webhook Jellyfin events to Ntfy, then your users can install the Ntfy mobile app on their phone and subscribe to events from your selfhosted Ntfy instance (or something close to that, I haven't set up Ntfy yet but had plans to).