Batboard

Hi. I have a CDM1550LS+ in UHF running (barely) on an LTR system. The Firmware in use is old, I mean really old...these are older, NOS radios.
What version should I look to upgrade to for "reliable" LTR operation? I read about LTR issues (missing calls, etc.) related to older firmware.

Any advice is greatly appreciated.

Thanks,

Bob