serial port - AT+CMGS returning 'ERROR 2171' -

at ok  at+cmee=1 ok  at+cscs="gsm" ok  at+cmgf=1 ok  at+cmgs="+44999999999" > hello test message [ctrl z] 

at point connection hangs 5-10 seconds.

+cms error: 2171 

i have been searching afternoon solution , yet find one. everywhere specifies errors above 512 'manufacturer specific'.