You can use SMPP protocol to push messages to the intended recipient. There are few open source implementation which provide smpp protocol implementation out of the box. It needs sms-gateway or you have to collaborate with some third-party provider. JSMPP has sample code to send sms(otp in your case), but it requires you to connect to some smpp-server, may be a modem which provide smpp interface.
Have a look at below links:
JSMPP- Click Here
PlaySMS- Click Here
