PCGs and SIM Server Systems are complete telephone call centers. 

Portable Call Generators (PCG) are man portable, easy to operate, mobile phone “relay” systems that uses a Single Board Computer (SBC) and a GSM Private Branch Exchange (PBX) gateway to create an untraceable conference call bridge between GSM mobile phones as well as between a mobile and a land line.  
 

These systems serve as a proxies that allow callers to place calls through the PCG and avoid dialing the called party directly. Bridging calls on the PCG creates two separate and disassociated call records in the carrier’s billing system which eliminates any record of a direct relationship between caller and the called party generated on the commercial mobile carrier.  Instead what is created is one call record from the Caller to the PCG and a disassociated second call record from the PCG to called party.