The Static method attribute only applies to methods of classes, not to local functions inside a script or function file. If you want GenerateSignal to be a local function defined inside DTMFGenerate.m that can be called by the main function in that file, DTMFGenerate, or by other local functions in the file then just define it using the function keyword.
In the mystats.m example on this documentation page, the functions mymean and mymedian are local to the file and the main function mystats can call them but they cannot be called directly by code outside that file.
