When selecting standby mobile phones, the most critical factor is Battery Capacity and Energy Density. Look for devices with at least 5000mAh to 10000mAh for extended standby times. Additionally, verify the Network Band Compatibility (2G/3G/4G/5G) to ensure the devices function in the target country's infrastructure. For industrial or outdoor use, prioritize IP68 or IP69K ratings for water and dust resistance, and ensure the Processor (CPU) is optimized for low power consumption rather than just high-speed performance.