This article describes the issue of branch devices being stuck in the u-boot mode, when a US Robotics modem is attached to the console port.
 

Branch devices get stuck in the U-boot mode, when a US Robotics modem is attached to the console port.

• OOB Management via a dial-up USR modem is configured and connected to the console port.
• When configuring a dial-up modem for OOB management on a SRX branch device, make sure that the local echo is disabled and the dumb mode is enabled on the modem.
• If local echo or smart mode are enabled, when the device boots up, the modem may be returning strings to the console; which causes it to halt the boot up process, and you will be left with the u-boot prompt. For more information, refer to KB12507 - Using a Modem for J-Series, SRX console for out-of-band management [juniper.net] .