your iphone could not be activated because the activation server is temporarily unavailable.

Please look at this --> If you can't activate your iPhone - Apple Support

If you are not the owner then there is no way that you can get into this iPhone unless the original owner shares the Apple ID and password with you. If you are lucky you can get into this iPhone if they (owner) accidentally remove the device from their apple ID.

If you are the owner then to open Activation Lock:- If it is Activation locked, read here --> Turn off Activation Lock - Apple Support Activation Lock for iPhone, iPad, and iPod touch - Apple Support

You need to have a live WiFi (or iTunes computer) internet connection in order to activate your phone. The reason is that the SIM only "teaches" the phone how to make voice calls. Activation teaches the phone how to use "Cellular Data" when out of WiFi range.

So, just connect to any WiFi signal and you should be activated in moments. Any WiFi source will do -- café, Apple Store, friend's home, train station ... anything. Good luck.