What is the current gateway for sending SMS with API?

Question

API

To confirm registration on the site's service, SMS began to be used. Not so much per day - 10-15 messages are sent. They stopped their choice at sms.ru. 50% of recipients are from Russia, the rest are CIS countries
And after 10 days - 47% of messages in the "not delivered" status. And Russian valid numbers. I contacted some users, tried to send from my phone and everything came without problems.
I understand the matter in sms.ru? If yes, then advise a normal gateway.
If not, then maybe they have some kind of lock on their phones?

Answer 1

SMS Center

Answer 2

SMS Pilot

Answer 3

try https://bsg.hk
In Ukraine from 24 kop.
In Russia from 20 kopecks.
It is possible to send to almost any country (more than 1100 operators).
There is no minimum quota for SMS.
Free test so you risk nothing