This scheme is some kind of RS-232 Driver/Receiver. Power is driven from COM-port.
Almost all digital devices we use require TTL or CMOS logic levels. That's why the first step when trying to connect the device to RS-232 port is transformation of RS-232 levels ack into 0 and 5 Volts. This is done by RS-232 Level Converters, like a Max 232.
There are also many variations of these devices. Other devices are available which use smaller capacitors and even some with built-in capacitors. (Note : Some MAX-232's can use 1 micro farad Capacitors). However the MAX-232 is the most common, and thus this RS-232 Level Converter will be used in our example.
78L05 - Linear voltage regulator.
This cable tested with: Alcatel OT, OneTouch db; Ericsson T10, T18s, T28s, A1018s, 888; Panasonic G450, G500, G520, G600, GD-70, GD-90; Sagem 612, 615, 715, 725, 730, 750, 820; Sony CMD-C5. Should work with most other phones except Nokia, Bosch.
Almost similar scheme, but with external power source.