@pfabrizi, you can use C# to convert time from Unix Epoch time to String Time.
In Splunk you can try the following Search
| rest splunk_server=local /services/deployment/server/clients | fieldformat lastPhoneHomeTime=strftime(lastPhoneHomeTime,"%Y/%m/%d %H:%M:%S.%3N")
@pfabrizi, that's really great!!! If you figured out solution on your own may be you can add the approach as Answer and accept the same to mark this question as answered.
Happy Weekend 🙂