sms.ready() returns value 3

Hardware Arduino GSM Shield
1

Hello,
can somebody help me on this? I can not find, what return code from sms.ready == 3 means - after sending an SMS.
The behavior is rather strange - when the RC == 3, the SMS is sometimes received by the target number, sometimes not..
Any advice?
Environment: Arduino Uno Rev3, Arduino GSM/GPRS Shield, Standard GSM from IDE 1.0.6

Code extract - stripped down to sample code:

  sms.beginSMS(newremoteNumber);
  sms.print(newtxtMsg);
  sms.endSMS();
  Serial.println(sms.ready());

Thank you in advance for help!