Telephony basics: numbers, PSTN, SIP, transfer, DTMF, voicemail

Learn how a phone call reaches your AI: a phone number (DID) on the public phone network (PSTN) is connected through a telephony provider (Twilio, Plivo, Exotel, Telnyx, Vonage) over SIP — the internet protocol for calls; a SIP trunk is the connection between the provider and your platform. Also: call transfer to a human (warm vs cold), DTMF (keypad presses like 'press 1'), voicemail and answering-machine detection for outbound calls, caller ID.