Purchased SMS gateway service from onnorokomsms .com.
Even though I have converted my cell phone into a gateway using Termux, sending costs 0.50/sms. And the gateway above charges 20% less per sms.
Bulk SMS can be purchased from my cellphone provider, but that costs 30/= per month at least.
And then there's the question of keeping a cell phone dedicated for sending the sms.
My conclusion would be if you are sending less than 100 sms per month, use a gateway service. More than 100? Build your own gateway.
My usage was to send tenants SMS on their payments and dues. Integrated and running. Works well. 0.40/sms.
The API is SOAP based. Was expecting REST API which are more raw and easier, but anyway. They have examples in Python, PHP, Java, .NET in their docs.
For recharge, you bKash any amount and add the transaction id on your control panel. That's it. No expire date for the balance. A positive nod of a company's good intention, my guess.