Comments by AzureAWS (Top 1 by date)

You can not directly send the messages using Web Application unless you use a third party platform to do so.

Please have a look:

1. Send an SMS Message with C# in 30 Seconds[^]

2. WhatsApp API - waboxapp[^]

For Cloud based solutions, you can check

1. Azure Notification Hubs Documentation - Tutorials, API Reference | Microsoft Docs[^]

2. https://aws.amazon.com/sns/

I've provided this based on your description of the problem. I may be wrong