You can build a call forwarding system using Twilio and Twimlets by following the steps outlined in this how-to guide. To start, you'll need a Twilio account, an email address to send voicemails to, and about 3 minutes of your time. You'll use Twimlets, small web applications that implement voice functionality, to create a call transferring system where one phone number rings multiple phones simultaneously, connecting the call to the first person to answer. The system will also forward calls to voicemail if no one answers. To set up the system, you'll need to generate a Simulring Twimlet, which dials multiple phone numbers and connects the call to the first person to answer, and a Voicemail Twimlet, which sends voicemails to an email address. You'll then copy the resulting URL and paste it as the voice URL for your Twilio phone number. With these steps, you can create your own custom call forwarding system powered by Twilio.