The MiMSMS (eSMS) SMS Notification Plugin offers many features for WHMCS as their billing system
The best place to start if you need help with a specific product is to contact the developer. All WHMCS Marketplace developers have both a website and support URL listed.
v1.0 Released Jan 01, 2019 Latest Version First release
A simplistic, free & open source hook allowing instant Discord notifications when an action is triggered.
MSG91 WHMCS SMS plugin gives you the power and flexibility to stay connected with your customers by sending them an SMS and OTP's for 2FA at the crucial steps that matters the most.
Netgsm Sms addon provides you sending customized bulk sms and creating sms templates.
More Advanced SMS System (Two-Factor Authentication)
Whatsapp Notification (Open source WHMCS addon module)