Audible tone after ignoring call

REPRODUCIBILITY (% or how often): ?
BUILD ID = OS VERSION: since 3.3.0.16
HARDWARE: XA2, X
UI LANGUAGE: na
REGRESSION: not sure, observed since 3.3.0.16

DESCRIPTION:

Sometimes, a signaling tone is heard after silencing or ignoring an incomming call.

PRECONDITIONS:

STEPS TO REPRODUCE:

  1. unknown, random

EXPECTED RESULT:

No sound.

ACTUAL RESULT:

Signaling tone is heard.

ADDITIONAL INFORMATION:

Observed when silencing a call or completly ignoring the phone.
I believe it is a “SIT”, " Special information tone", the “RO’” or “IC” in


Once I saw the message “Network error”.
Frequency: much less than 1 in 10 calls.