Thanasis mailed me about this issue, maybe I can clear some things up.

First of all we do not use mobile phones, but actual GSM/UMTS modems.
Usually these are models from Siemens/Cinterion namely MC35i and HC25
which do immediately respond with "OK" to "USSD" before returning the
actual USSD response.
We mainly use the USSD function in order to check the balance of
prepaid cards which has proven to work stable for months with cards
from different European countries, although one German provider
recently switched to respond via SMS to an USSD balance query for
whatever reasons.

I do however see your point, but I suspect we will need to use custom
ATHandlers in order to work around this issue as the immediate
response to the AT commands is given by hardware.
One might argue about what the behaviour of the default ATHandler
should be, though. This would especially be true if it turned out most
modems/phones did behave in the way the one you use does.

On 18 Aug., 17:23, Jeff Gehlbach <[email protected]> wrote:

> TIGO Honduras (CelTel):
> #123# = check account balance
> *327*DEST_NUM*NN# (*DAR) = give NN Lempira (HNL, Honduran currency) of  
> prepaid balance to DEST_NUM
>
> This last code is interesting in that it keeps the USSD session open  
> beyond the initial network reply, requiring further action from the  
> user.

We stumbled across those, too. These are interactive sessions which
are specified in the corresponding standard. So far the only provider
known to use those was Vodafone on checking the balance of a card.
In order to avoid complications by this my original code simply
terminates this session by sending an "ESC" char to the hardware.
There is a comment in the code explaining this.

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