hye firdaus , from ur coding , it seem like its already correct . it maybe problem between ur pic and gsm.. and i dont clearly understand ur project without a schematic . do provide it . I think ur problem is because PIC provide 5v and Most GSM operate in 2.8v to 3.9v . both device cannot comunicate and if u do directly from pic to gsm . it may broke ur gsm . so i suggesting u use LOGIC LEVEL CONVERTER ( LC04A) . it just 2$ . and remember to use TTL pin in ur gsm and not using serial comunication pin . in my gsm ( sim900a) the ttl pin have 6pin … the two top provide 3v , middle is 5v . and bottom is GND and VDD . when ur using LOGIC LEVEL CONVERTER . use 3v TTL pin Gsm to LLC LV . and 5v from pic to LLC HV. '-' on both lv and hv should be in the same GND. for more detail contact me .