mncc_builtin: Properly reject DTMF
As per TS 23.014, a GSM MSC must implement mobile-originated DTMF
generation. We gate the DTMF signalling messages to MNCC, and expect
the external MNCC handler to deal with it. However, the internal MNCC
handler simply ignored such singalling messages, rather than rejecting
DTMF altogether.
It turns out failure to respond to START DTMF will cause some phones to
behave in interesting ways, particularly with modem
firmware v6.01.00, see https://osmocom.org/issues/1817). In this case
the phone is not able to release the call as the pending response to the
START DTMF is probably keping a reference or lock of some sort.
Change-Id: I336f0cd0a6396b522d228479a417fd4d606157ac
1 file changed