Asterisk and three yeastar gt800

i have three TG800 . How I can configure Asterisk with three TG800 if I have 24 out IP calls to GSM and Asterisk automatically selects the TG800 and the first GSM SIM free? How Asterisk can see the three TG800 like only one device with 24 gsm card? thanks

With that requirement, you would be better off getting a T1/E1 connection to the cellular network operator. You will be tying up a large proportion of the capacity of one cell with traffic that would be better not going over the air at all.

If the devices return a suitable congestion indication, you try each one in turn until ${HANGUPCAUSE} indicates either a fatal problem (no answer, busy everywhere, etc.) or you get through. You could also use global variables or queues to load share them.