Yes, you can set it as high as +12. There is conflicting information between the Admin guide, spec sheet and fly-over help on the expert config page.
I just tested it at +10, and it is loud as hell. Disclaimer: I have a slight hearing loss, and my ears are not calibrated decibel meters, hence my unscientific unit of measurement, "loud as hell". In other words, 12 may not actually be louder than 6, but you'd need instrumentation to determine that, and it depends on the impedance of the telephone microphone.