Voice over Internet Protocol (VoIP), is a technology that allows you to make voice calls using an active Internet connection instead of a regular Mob. SIM or phone line.
DID (Direct Inward Dialing) is a virtual extension number of an ISDN or SIP trunk, also known as DID number or sometimes a Local Number of any particular destination.
A toll-free phone number is one that you can dial without having to pay for the call. E.g. TFN starts with 1800 in India and 800 in the USA.