Why did I hear a ring back tone instead of the busy tone while calling from a local FXS the voice card on the opposite Router2, which was currently busy?

Q: Why did I hear a ring back tone instead of the busy tone while calling from a local FXS the voice card on the opposite Router2, which was currently busy?

A: Networking as follows: FXS-Router1-------------Router2-FXS (or FXO)

This case may exist. The ring back tone is sent by Router1 (most prompt tones are sent by the local end). By default, the non fast start-up mode is used. In this case, the debug voice ipp all command of local end is enabled when you make a call. In the debugging information, if the H245 message of the opposite party replies the Alerting message, the local end plays the ring back tone. If Router2 replies the CCBRelease message, Router1 plays the busy tone for the local end upon receiving it. Therefore, if Router1 uses a Huawei product, but Router2 may not use a Huawei product, Router2 returns the H245 message during interworking. To let the opposite party play the tone, both Router1 and Router2 shall support the fast start-up mode, and partial information of H245 is carried in the H225 message.

Thus, you can hear the busy tone when the opposite party is making another call!