While investigating why we were being charged by our SIP trunk provider for calls made by Lync synthetic transactions, I discovered ms-skip-rnl. By adding this to the SIP or Line URI Lync will not try and match the number to an internal assigned number (by way of reverse number look up).
This explained why the synthetic transactions where dialing out the SIP trunk only to come back in again. This makes total sense as it is truly testing that an external calling can dial in. Only trouble for us is it was racking up a $100 worth of calls every month!!! So be aware Lync synthetic transactions may incur call costs depending on your SIP trunk provider.
This discovery of mine could also be applied to a LineURI if you wanted to push all calls out to a gateway or phone system. Bare in mind though that the call will never ring on the Lync end point so the gateway or other phone system will need to take care of that.
Matt Ellis has got a pretty good article its use with a Cisco Call Manager – http://mattellis.me/lync-2013-cucm-ms-skip-rnl-sip-uri-dialling/