Right, we ping the servers every minute. Since we charge a one-time fee the credits expire after a year, but the service is scaleable. To answer your question I'll give you some more context:

The architecture uses scalable AWS serverless components (Lambda, SQS, DynamoDB) and is well-suited to handle a large increase in monitored endpoints. The primary scaling mechanism is the automatic concurrency scaling of the Lambda functions processing messages from SQS queues. Should we scale to 10,000 endpoints we do expect some bottlenecks that would require optimizing i.e. increasing lambda timeouts/memory etc. but we'll cross that bridge when we get to it.

For the actual sms sending our numbers can send up to 100 sms texts/second.