[asterisk-users] dtmf detection

[Rilawich Ango]

Hi,
  Below is my case.

phoneA (PSTN)
phoneB (SIP)
phoneC (PSTN)

phoneA --> asterisk --> phoneB
phoneA (music on hold), phoneB --attended call transfer--> phoneC
phoneA --connect-- phoneC after phone B hangup
phoneA type some keys in keypad but phoneC always has wrong dtmf detection.

In my case, I would like to know any factor that will cause the wrong
dtmf detection.