From a phone connected only to your PSTN line, you can make a GV call by dialing your GV number. At the main menu, press 2 to make a call. This is most useful if your PSTN service has unlimited local calling and your GV number is local to you. (Your PSTN number should be one of the forwarding numbers on the GV account, but you can of course set GV to not actually forward there.)