We run a web service for Smartphone and old-fashion mobile phone. It is one application but screens are modified a little for each device.
For Smartphone, we implement the analyticsjs by Google Analytics (Universal Analytics). But JavaScript is not running on the old-fashion mobile phone. So we plan to implement Measurement Protocol on our Server.
Could we use one tracking id - like a "UA-XXXXX" for Smartphone and old-fashion mobile phone both?
Tracking ID is the identifier of Google Analytics data set. So if you use the same ID your data for smartphone and feature phones will be collected within one set and you can build reports for both platforms at the same time. Usually, this is a desired set up. Consider using dataSource analytics field for detecting what platform generates the data https://developers.google.com/analytics/devguides/collection/analyticsjs/field-reference#dataSource