Fix for dropped calls in Argentina (Denwa)

Allow receiving metering pulses with forced release

Not sure if what they are seeing is really a metering pulse or
a glitch in the line leading to 0x00 CAS pattern, but pretending
is a metering pulse will avoid dropping the call

We tried using cas_persistance_check timer to 500ms, and that seemed to solve
the issue of dropped calls, but incoming calls would not work anymore, most likely
because the other end time out for the seize is less than 500ms