Asterisk *can* work with OnSIP. Generally, if you are using Asterisk, you will want to use our PSTN Gateway product and register with sip.jnctn.net. However, there may be few, very special circumstances where you would want to incorporate OnSIP users with Asterisk. To do this, you must be running Asterisk 1.4 or later.
In sip.conf, your register statement would be:
; ; The "general" context should already exist in sip.conf ; Add a line to register with with Junction Networks ; [general] register => MY_USERNAME:SIP_PASSWORD@MY_DOMAIN.onsip.com
Alternately, in the OnSIP interface, you can change your SIP address (username) to equal your auth username.