Gammu and Wammu are free software projects. As such you can get some support from the community which is behind the project. The best way where to start is usually to check documentation whether you problem is not just a configuration issue or some well known situation.
If you are looking for list of supported phones, you can look into Gammu Phone Database. You can also share your experiences there, but if you had problems, please report bugs and do not write about problems only in phone database.
You can also ask related questions on Stack Overflow.
If the documentation does not help, you can try to look at mailing lists, first looking in the archives if somebody did not face the problem. You can also try reaching us on some online communications channels such as Jabber or IRC.
Once you think you have hit a bug or want to file a feature request, please use a bug tracker to report them. Please be as descriptive as possible and do not forget include necessary information to reproduce the bug such as debug logs.
Running some business application using Gammu, you might be interested in getting better level of support. Just contact the developers and try to ask them for this.