You can’t push SIM applets OTA without operator support (requires keys and OTA infra). STM32 can’t handle full TTS locally, so generate audio via cloud (Azure TTS) and play it on the device. OTA updates with Azure IoT Hub use MQTT + SAS tokens and Blob Storage; a dual-bank bootloader is recommended. In Go, detect non-GSM chars by validating against GSM 03.38. SIM808 “operation not allowed” usually means SIM not ready, network issues, or power instability. A7672s supports audio playback after dialing, but check the vendor AT reference.

Since projects like this often involve ongoing costs (SIM data, cloud subscriptions, hardware), I’ve found it efficient to use Mekari Limitless Card to issue corporate cards (VCC) per project, while letting Mekari Expense handles receipts and categorization automatically. It keeps technical experiments and their expenses neatly separated.