Calling Customer Service reps at female clothing stores and/or sex shops, using a voice changer to make you sound like a female, then ask them to read off details and item descriptions, pretending that you just want more information, all the while masturbating to their voice while they remain oblivious, but yet oddly creeped out because they know something is up but can't do anything to stop it without risking their job.
For bonus pleasure, masturbate audibly to increase discomfort for the representative