3G modem: simultaneous sending SMS and having active Internet connection

Question

I am writing client-server application.
The client sends some messages to the server by using different transports: SMS and Internet (VPN, TCP server).
On the client's PC I have 1 3G modem which I use for both: sending SMS and establishing Internet connection.
I connect to Internet via ubuntu's Network Manager.
For sending SMS I use smstools3. Actually it doesn't matter what tools I used.
I found that it had been impossible to send SMS if there had been active Internet connection established via the same modem.

I tried to connect to Internet without the Network Manager. I manually setup pppd (with no lock param) using the manual from http://www.techytalk.info/manage-gsm-mobile-broadband-connections-without-network-manager-ubuntu-mint-debian/ . The connection worked fine, but it did not solve the problem.
Then I tried to use different modem's ttys for sending SMS and connecting to Internet (my modem has 2 ttys). It also did not help. Looks like only 1 tty of my modem can be used to do the job.

Now I have only 2 ideas:

1. Don't do the things simultaneously. Separate them in time: start smstool daemon, send SMS, stop the daemon, establish Internet connection, send msg, stop the connection.
2. Try to use GSM multiplexing like described here gsm mux I am not sure about the result because the mentioned project (gsm mux) looks dead.

Do you have any other ideas how to solve the problem?

BTW, I use the following things:

• OS: Ubuntu 12.04 LTS
• Smsd: 3.1.14
• Modem: Huawei E800 ExpressCard

Answer 1

Finally, I found the solution.

Huawei E800 couldn't be used. Instead, I used E369. That modem has 3 ports (3 ttyUSB in /dev/). If I use the 1st tty for the Internet and the second for sending/receiving SMS, then the problem disappears. Note that E800 also has 2 ttys, but it doesn't solve the problem.

I think I known how to figure out whether a modem can be used for simultaneous SMS and Internet. Need to send "AT+CMUX=0" to the modem. If it answers that the operation is not supported, then you have to try different modem.

Also I met 1 more problem on Ubuntu 13.04. If I used the ubuntu's NetworkManager to set up Internet connection, then it blocked my modem. So, the modem couldn't be used to send/recv SMS. I found the solution - didn't use NetworkManager. Instead, I set up Internet connection with wvdial. It solved the problem.