Operating system name and version: Windows server 2008 + install smsd how service
Version of smsd: 3.1.14
Smsd installed from: sources
Name and model of a modem / phone: mc35i
Interface: com -> usb

Good afternoon. I'm sorry for my eng, use translate. I have some exemplary script in bash:
if [ "$1" = "RECEIVED" ]; then
sendsms +number "Start"
->sleep 30
-> here some on bash code, witch need ~10min to complete
->
sendsms +number "Done"
fi

If i run, like:
/home/my/script RECEIVED /var/spool/sms/incoming/GSM1.123fasqw
I have:
1 - sell sms on my phone
2 - next run execution bash script
3 - sell sms "Done"

If i send sms from my phone to modem, i have this:
1 - forward before script has completed
2 - take 2 sms: Start and Done...

Why is this happening? Thanks.

If you use the same modem for receiving and sending its handler may be blocked until the script completion and because of that the handler doesn't check outgoing queue. (Just a guess. I haven't taken a look at the source code.)

After the script completion the modem handler finds both two messages in outgoing queue and sends them immediately.

If it is the case one probable solution can be to spawn a background job from your script, like the following:

if [ "$1" = "RECEIVED" ]; then script_that_does_long_job.sh & fi