You are allocating a new QProcess every time on_InviaSMS_clicked() is being called, without destroying it afterwords - not good - your this is a memory leak.
The best would be to initialize it once and start and close new process on the same object.
Or, if you want to allocate a new object every time you need to delete it when you don't need it anymore.
But I don't think this has to do with your problem.

What happens if you add a '\n' to the last string you send?