Can I test sender IDs in the Sandbox Account before using them?

Yes, but with limits. The Sandbox Account at https://unify.smsgateway.center provides a default sender ID for testing. You can’t add custom IDs there—upgrade to a paid plan to use ones from “Manage Sender ID.” Test basic SMS flow in Sandbox, then manage IDs fully post-upgrade. See more at smsgatewaycenter.com/sms-types/.