Hello,
I have previously used a relatively cheap OTP provider for an android application.
My target market for the android app was East Africa, but the OTP provider serves all of Africa, and other continents too(partnership with gsm carriers)
The provider also gives out free OTP sms every month,so you wont pay if your volume is low.
get in touch so we get started.
If you happen to require assistance in integration of the OTP to your project, we can also talk about that(OTP integration to android,website)
Looking forward to hearing from you.
Regards,
Ben