Contacts are normally saved in the internal storage/phone book of the Android device but factually, some of the contacts are copied from the SIM card for usage in the Android device. Though you may use the contacts, right from the SIM card, but the using the calling contacts from internal contacts is always better and quick.
You may do so at your end, easily, but if you feel need, see the instructions.
Instructions
1: Tap the icon of the ‘Phone Dialer’ app at the Home screen of your Android 5.1 Lollipop device to open up the ‘Phone Dialer’ screen.
2: At the ‘Phone Dialer’ app screen, tap the settings button, placed on the top right corner of the screen. It will open up a popup menu.
3: Tap the ‘Import/export’ in the popup menu. It will open up a prompt with 4 options.
4: Tap the ‘Import from SIM card’ in the prompt. It will open up ‘Select contacts to import’ screen. 
5: Here, you may select the contacts to import from the SIM card to the internal storage.
6: But, if you want to import all contacts from the SIM card to the phone book/internal storage, just tap the button, lying in the top right corner of the screen. It will open up a popup prompt; ‘Import all’. 
7: Tap ‘Import all’ and all contacts in SIM card will be imported to the internal storage of the Android device.
That’s all!