VoIP

my biggest confusion on the voip thing is that , assuming i make a call from my PC or any telephone equiped with VoIP and another phone maybe that does not support the VoIP features say an analogue telephone for the home , is there any gateway that route the traffic to the PSTN of the telco on that analogue telephone or i don’t need anything like that , i’m looking at this fro the design perspective of a VoIP .
TIA
Hashiru