V23 Softmodem quality


I am new with Asterisk and my project is to setup a V23 (only 1200 bauds) server with Asterisk using a softmodem.

Already installed Asterisk 21 with soft_modem module.

All configured : when calling, Asterisk answer and I can connect with my (old) V23 modem.

My problem is the quality transmission of data : sometimes, the data transmission quality is relatively good, but most of time pretty poor. I know it could be an issue with VoIP.

Of course, the configration of data bits, stop bit, parity is correct.
I am using G711 alaw codec (allowing only this codec).

The config of extension is:

exten => s,1,Set(VOLUME(TX)=7)
exten => s,n,Set(VOLUME(RX)=25)
exten => s,n,Answer()
exten => s,n,Softmodem(“”, 8183, r(-28)t(-40)v(V23)ld(7)s(1)e)
exten => s,n,Hangup()

I made a lot of tests changing the volume parameters and found that these ones are the better (or the less bad).

Do you know if there are other parameters I could change to improve the quality of trnasmission ?