After finding that Sonim's software uses GPL licensed Gammu, the obvious step was to ask them for sources of the software. They quickly transferred their responsibility to company which made software for them - A&W (Anwsoft).
It took some iterations to explain them that I'm pretty sure that their software is based on Gammu and finally they gave up pointing to some external contributor for being guilty of this:
Due to the product was developed years ago, and the source code was bought from a SOHO programmer on the first beginning, so we didn’t notice it included some open source code inside.
The good thing in the same thing was that they were willing to cooperate:
We are willing to cooperate with you to follow GPL License and provide corresponding sources.
As it turned out in upcoming communication, they didn't have (and still don't have) idea what GPL license contains and what they should publish. But that's story for the next post.