Security

Host can be “dynamic”, accepting registration from any IP or it can be assigned to a specific IP address. In this way, no registration is needed.
Insecure allows the peer to be authenticated using the IP.
Transport permits to use a different transport for the signaling. If TLS is selected, it is needed to add a certificate to Asterisk. Please check the Asterisk FAQ for special configuration.

RTP Encryption provides encryption to the RTP (audio) part. The key is transmitted over the SIP channel, so it will be useless to use it without setting the transport to TLS.
Send RPID and Trust RPID permit to specify how to transmit the CallerID information from this extension.
Outbound Destinations permits to restrict the numbers the extension can dial. In other words, the destination allowed can be restricted. For example, the phone placed in the kitchen of the office can be restricted to place international calls. The Outbound destination can be:
All Allowed: Allowing every number
All Prohibited: The phone cannot place outbound calls
Allowed if matches: The call is allowed if the number dialed match the Regex associated
Prohibited if matches: The call is prohibited if the number dialed match the Regex associated