as you already assumed, it is a SIM-Card slot. The small white icon in the battery bay shows you how to insert the SIM-Card correctly. It is also covered in the OperatignManual of the LIFEBOOK P701 on page 45 (page according pdf application)http://support.ts.fujitsu.com/indexdown ... 05E2A43BCA
But please note. The SIM-Card slot is always built-in, but to use it, it is mandatory that you ordered your device with an UMTS-Module. If you are not sure you can check this in the devie manager under network adapters if a UMTS-Module is build in.
Can you see a Sierra Wireless MC8305 WWAN module there?