I've solved the problem changing the flash time in the zapata.conf file,
I've set:

flash = 200 (the defualt was 750 ms)

in the extensions.conf the code is for example:

exten => 42,1,Flash()
exten => 42,2,SendDTMF(42,250)
exten => 42,3,Hangup()

now the transfer with flash works correctly

I've tried to transfer a call using the Flash command, but with my configuration it doesn't work.
I have a traditional PBX connected with a zap channel to Asterisk that acts like an IVR:

TELCO line --> traditional PBX (FXS) --> (FXO) Asterisk

From the TELCO line I can make a call to the traditional PBX and reach Asterisk, the IVR system on Asterisk answers the call and I can dial an extension (for example 42 that is on the traditional PBX). In the asterisk dialplan I've set to transfer the call using Flash() like in this example:

exten => 42,1,Flash()
exten => 42,2,Background(silence/1) wait 1 second for the traditional PBX
exten => 42,3,SendDTMF(42,250)
exten => 42,4,Background(silence/1) wait 1 second for the traditional PBX
exten => 42,5,Hangup()

When I dial the extension 42, the phone 42 on the traditional PBX rings but when I answer there isn't communication with the call from the TELCO line and after a few seconds the line hangup.
Here you can see what happen in asterisk CLI console:

      Executing Answer("Zap/4-1", "") in new stack
   — Executing BackGround("Zap/4-1", "a_suoni_plink/menu_esterno2") in new stack
   — Playing 'a_suoni_plink/menu_esterno2' (language 'it')
 == CDR updated on Zap/4-1
   — Executing Flash("Zap/4-1", "") in new stack
   — Flashed channel Zap/4-1
   — Executing BackGround("Zap/4-1", "silence/1") in new stack
   — Playing 'silence/1' (language 'it')
   — Executing SendDTMF("Zap/4-1", "42") in new stack
   — Executing BackGround("Zap/4-1", "silence/1") in new stack
   — Playing 'silence/1' (language 'it')
   — Executing Hangup("Zap/4-1", "") in new stack
 == Spawn extension (incoming, 42, 5) exited non-zero on 'Zap/4-1'
   — Hungup 'Zap/4-1'

I've tried the following changes to the dialplan in my example but transfer still doesn't work:

- I've tried to use wait(1) instead of Background(silence/1)

- I've tried without Background(silence/1) or wait(1):

exten => 42,1,Flash()
exten => 42,2,SendDTMF(42,250)
exten => 42,3,Hangup()

- I've tried without the Hangup() instructions at the end

Has anyone the same problem like me and any suggestions?

whe i try the recall button, this is the only thing I get

Feb 24 00:19:48 DEBUG17030: Exception on 10, channel 1
Feb 24 00:19:48 DEBUG17030: Got event Pulse Start(14) on channel 1 (index 0)
Feb 24 00:19:48 DEBUG17030: Exception on 10, channel 1
Feb 24 00:19:48 DEBUG17030: Got event Event 65585(65585) on channel 1 (index 0)
Feb 24 00:19:48 DEBUG17030: Pulse dial '1'


any ideas

This isn't the behaviour I have seen... I can hit flash a few times and switch between the calls. If I hang up, the other two calls are conferenced together, but I'm out of the conversation. I tried it a bunch of times.

any special context needed to accomplish this?

I am able to transfer calls on the BT102 by pressing the tranfer key ,enter the extension, then push the send key. You'll also be on the line, you can hangup or have a three way call.