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Communication systems require that
a person receiving a call be able able to charge the caller when they are from
different administrative domains. This is necessary for making fair exchanges of
service between two different communicating parties and is a major strategy for
reducing the viability of SPAM. This draft proposes an approach for doing this
in SIP. The approach relies on a third party to act as a payment service
provider and is optimized for very simple, low value transactions. It does not
provide the full range of services that are desirable in typical online trading
systems.