REPLACESYSBAT W/POWER ON Message on my Partner Phone

This is the system telling you that it's time to replace the 2 AAA Batteries that retain the programming when the power goes off or you unplug the system.   

Step 1:  Obtain 2 AAA Batteries

Step 2:  Locate your Partner ACS.   It looks like this, though keep in mind it may be in a 2 slot or 5 slot carrier with other modules.

partner acs find and replace battery