I've got a few stanzas in /etc/asterisk/custom/iax.conf that allow a few of us to connect via smart phones. 

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Setup of Asterisk Server for iaxRPT Connections

If you properly answered the questions at install time, the Asterisk/EVB w/app_rpt installer automatically enters the necessary information in the configuration files "iax_rpt_custom.conf" and "extensions_rpt_custom.conf" for your node . Otherwise you will have to edit or create and include these files manually into your dialplan along the lines of the example link below.
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This seems a bit vague.