Always a busy signal on a smartphone when using a Multi-SIM for the same number in a notebook

Last year at work, we had the curious case that many of our notebooks, which were using a SIM for their UMTS/LTE modems, caused a smartphone with the associated Multi-SIM (same contract, i.e. same telephone number) to respond with a busy signal as soon as the notebook was powered on (didn’t even had to have a connection established via their WWAN modem at that point!).

As it turned out, this was a problem with Multi-SIM cards from Deutsche Telekom in combination with the Huawei LTE modems in many of our notebooks; a well-known problem, but not really fixed, neither by newer drivers or Deutsche Telekom (SIM/service).

A manual solution is to change the mode/setting by writing to the virtual COM port that the LTE modem is using under Windows 7.

Show current settings

To display the current setting, use this command:

echo at^syscfg?

Change settings

Put this in a batch file and run it on the affected machine:

echo on
@mode COM3 BAUD=115200 PARITY=n DATA=8 STOP=1
echo at^^syscfg=2,2,3fffffff,1,1 > COM3
timeout /t 5

Same things to note:

  1. The COM port may differ: We had multiple Lenovo ThinkPads, and on certain models, COM3 (see above) was used by the Huawei modem, but on another model, it was COM 5. Look in the Windows device manager for more details and act accordingly (e.g. via brute force: run multiple, adjusted batch files).

  2. The AT string above is only for a Huawei modem. Hardware from other vendors need different values. Some examples of it are listed in the forum threads linked below (only available in german, sorry. But the code snippets are easy to recognize…).

  3. In some instances, this commands had to be run twice or even three times before the system applied the changes. Maybe a matter of timing; I didn’t spend time investigating it because that happened only a few times.

Background information

Here’s some more background info about the whole issue (in german):