Install 2 usb voice modem usb huawei

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hello everyone, I followed an Italian guide (link below) for installing an usb "voice modem usb " to have a gsm trunk in my switchboard, and all work. The thing is that I would like to have 2 gsm “voice modem” but I don’t know how to menage that, so I would be really grateful if someone can help me to manage 2 usb “voice modem” for the input and output calls. Thank you very much

link: https://www.google.it/url?sa=t&source=web&rct=j&url=http://www.lovisolo.com/asterisk/Asterisk-chan_dongle.pdf&ved=0ahUKEwipisHOyfXLAhXDmg4KHQIGCGcQFggjMAI&usg=AFQjCNEJ1ihqQtFg0K4dDTDpQgVJypSeyg