Do you want to change functionality (other than available admin settings)? Or do you just want to change the text that is output in the "call for price" situation?

All output text is defined in language files; you can find the file to change by using admin > Tools > Developers Toolkit. Paste the text you want to change in the lower left box, select all files and search. Among the results will be the file and line number where this is defined. You can then open that file in a file/text editor like Notepad++ and change it as desired.