Sending DTMF Flash Event to Transfer Calls


There are 2 analog PBX located in 2 different buildings. It's impossible to connect the 2 PBX by copper cables. Therefore, we need the TA VoIP gateways to interconnect them. Also to achieve the goal to transfer the call by Flash Event. 


Application Scope

Product: Yeastar TA  FXS VoIP Gateways, Yeastar TA  FXO VoIP Gateways

Firmware version: TA400 - or higher, TA410 - or higher


Sample Scenario

TA400 and TA410 are interconnected by SIP trunk (Service Provider - Peer Type). As well, there are analog PBX connected on both devices by analog lines.

Practically the 2 goals in this scenario:

- The extensions on PBX1 and PBX2 could be called each other.

- The extension on PBX1 could transfer the calls by pressing Flash key.


Assuming extension 201 belongs to PBX1, and extension 101 belongs to PBX2.

  • Call Flow Sample

1. Caller 1234 is on the call with 201.

2. Extension 201 tells caller 1234: I am not the person you are looking for, and I will transfer the call to extension 101.

3. Extension 201 tells caller 1234 to hold on, and he presses the Flash key on his phone

4. TA400 receive the Flash signal from PBX1, subsequently, TA400 send SIP Flash DTMF Event (RFC2833) to TA410 through the SIP trunk.

5. TA410 receives the Flash event, and then sends the Flash signal to PBX2 through its FXO port. And the FXO port hasn't been released.

6. Caller 1234 has been on hold.

7. Extension 201 calls to extension 101. 101 answers the call and starts to talk.

8. Extension 201 tells extension 101: there is someone on hold looking for you. Please deal with it. Or Extension 201 hangs up the call after hearing the ring back tone.

9. Extension 101 accepts. Extension 201 hangs up the call.

10. Caller 1234 and extension 101 start to talk.




Eanble the option "Send Hook Flash Event".



There is no special settings.

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  • 0

    Dear Xie,

    thanks for very nice article and clear explanation!


    I have question for small difference in configuration:

    1. Instead PBX1 and TA400 there is Yeastar S20 (or any other), SIP connected to TA410.

        I understand that ext 201 is analog phone.

        Is it now possible for S20 to send SIP DTMF Flash event, so that TA410 can generate Flash signal?

    2. If ext 201 is SIP phone on S20,  the same question.


    If possible, please explain procedure flow and S20 programming!


    Thank you, Nebojsa

  • 0

    Min flash: 300

    Max flash: 800

    worked for me

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