Embedded app - Call startTime format is as "1591774064578". when we access this from the Windows.Phone.CallLogs.Calls object. Please help on the format

It is the timestamp rather than the time. If other time fields in that call log is the time, then it is a mistake that the app does not convert the timestamp to the time. If all the time fields are the same or that is the only time field, then it might be logged as a timestamp (based on UTC time) on purpose to make it easy to convert to a local time. You can convert the timestamp to a time e.g. in Javascript as follow:

var startTime = new Date(callStartTime).toISOString()