You can choose different ringtones on your HiHi depending on whether the call is an internal or external contact. To change the ringtones slide down the menu drawer in the top right of the screen, press settings on the left-hand side, select sound then press phone ringtones.
Within the ringtones tab you will see two columns for internal and external calls, these columns relate to the tone that will play when a call is received. Pressing on each ringtone will play a sample. To set the ringtone you like most simply press ok.