NACK/MALFORMED in sending SMS via AT interface

Hi all,

I asked this question in users mailing list and did not receive any answer hence I tried to ask in in devel list. 

My problem is that I get NACK/MALFORMED error while sending SMS using GSM modem. It used to happen a few times in the past but now we face it several times each day.

I searched a bit, and noticed that NACK/MALFORMED is generated in gw/smsc/smsc_at.c. It seems that after sending AT+CMGS whether '>' does not appear or after sending message modem does not return OK msg. Debug info shows that '>' appers but after sending 'Ctrl-Z' it just returns 'ERROR'.
Has anyone encountered such a problem? Maybe I should insert a delay before reading '>' and 'OK'?

Sincerely,
Amin