What is developer SMS API?

For communicating server to server DEVELOPER API  this authentication method is used

API allows automatic sending of text messages like Payment Due reminder message. SSD Web Solution Provide API for free of cost. Sign up for the free trial account.In the trial account, you can get API & Sample code use that & integrate into your system to send real-time text messages.Messages can be sent event based, time-based and can be user initiated.

Bulk SMS
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I don’t want to renew my account, then what should I do?