The config on server B has no IP based matching, so it is using user based authentication. The default for user based authentication is to match the endpoint based on the user portion of the From header. There is also the possibility of using the authentication username by using “auth_username” in the endpoint identifier order. Finally for callerid P-Asserted-Identity or Remote-Party-ID should be used, using the send_pai and send_rpid options respectively. To trust it you would need trust_id_inbound set to yes.
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