[asterisk-users] Busy (congestion) signal and cell phones

[Mark Gimelfarb]

Hello, all!
I've noticed a peculiar situation and I am hoping someone can shed  
some light on it for me. We have an Asterisk (1.4.18 ) box talking to  
the world via Zaptel on a PRI from a telco (USA). I have an extension  
that returns busy signal (fast-busy or regular busy) (using US tones).  
When I call from a landline or from another PBX, I get a busy signal,  
just like I expect. But when I call from a cell phone, the cell phone  
terminates the call as soon as connection is established. I've tested  
several cell phone models from different providers in the US. Same  
thing happens with calls coming from Gizmo.

I manually changed the tones I send back (with Playtones) to mimic  
Austrian busy tone (picked the first one in the list from  
indications.conf) . Now, from the cell phone and Gizmo alike, I get  
busy tones. So, my questions is:

why do cell phones and Gizmo both detect busy tones and terminate the  
call? Is that a standard behavior? Why don't landlines do that?

Thank you in advance.

Regards,
Mark G.