What are some web based free audio conferencing solutions where participants can join without registration?

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Our company http://Onepgr.com offers a web-based solution where the session host can start audio call  (using PSTN or VoIP) and attendees can join the conference call (again, using PSTN/VoIP). Only the host user needs to have an account and either invite the attendees to the session, or simply share the Guest URL with attendees.