Is it possible to see the IP address of the stolen phone? How could I
If you want to get it from Apple, you'll have to subpoena them for it. To get the address and owner info of that IP, you'll need to subpoena the ISP or carrier.
Apple will tell you where your phone is, but they will not tell you give you the details of who who owns those things without sufficient a sufficient legal basis to do so.
You can get your IP from different services, like gmail, dropbox, etc. When these services periodically check in, they can tell you from what IP address it checked in with.
For instance, I use Outlook.com and using their Activity feature, I can get the IP of my last sign in:
It will not, however, tell you the actual address, to get that, you'll need to subpoena the ISP. The location data that they give you is not the actual location of your phone, it's the location of NOC that handed out that IP.
All of this information is obtainable. Police departments can (quite easily) obtain IP, GPS coordinates, cell tower identification, etc. However, the costs of doing so often outstrip the value of the device. Other peoples identifying information is protected (even if they did steal your phone) and you (or the police) need to go through the legal process to obtain this info (lawyers cost money even if they work for the city).
I'm not saying all is lost, if you can help them with location info and enough probable cause for the police to search (assuming you're in the US) and you can actively ping the device that points them to location (i.e. you sharing Find My Phone data with them real time), they can execute a search. The "Make a Sound" feature can help them "proof positively" identify your device for further action.
It's far better to use the Find My... geo location data than an IP because you don't need to process any legal paperwork to (accurately) find out where you or your device is or has been.
Get this data and file a police report.