Lunch gammu on demand

Hello, i will lunch gammu on demand, only if a create a sms, after, il will exit gammu.
The reason is i don’t use a same modem (ttyUSB2) for chan dongle on asterisk (for receive call or take a call) and send a promotionnal sms.
If gammu is started, i have a busy or occuped device on asterisk 16.
Any people have a solution to use a tty for 2 application in same time or it’s possible to change the script to send a sms by playsms for include a start gammu service and what the sms send, close gammu, thank you