Specifications

SIM900 AT Commands Set
SIM900_ATC_V1.01 19 23.06.2010
the call is established. The setting is controlled by AT+COLP. Factory
default is AT+COLP=0, this cause the TA returns “OK” immediately
after dialing was completed, otherwise TA will returns “OK”,
“BUSY”, “NO DIAL TONE”, “NO CARRIER”.
Using ATD during an active voice call:
z When a user originates a second voice call while there is already an
active voice call, the first call will be automatically put on hold.
z The current states of all calls can be easily checked at any time by
using the AT+CLCC Command
2.2.4 ATD> <n> Originate Call to Phone Number in Current Memory
ATD><n> Originate Call to Phone Number in Current Memory