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It should be possible to create RingMe buttons for any externally reachable number that can be dialed. I would also suggest that instead of implementing the RingMe in the Admin Portal rather than for each extension. That is difficult for an administrator, who is the likely person to generate the buttons, to have access to various RingCentral extensions. My thinking is that you should be able to configure a RingMe button simply by clicking on the number, and then the RingMe configuration should be surfaced someplace for an Admin to create.