The GRUU SIP extension  has been recently added to OpenSIPS trunk and it will be present in the 1.8 release.
A GRUU ( Globally Routable User Agent URI ) is specifically designed to implement reliable routing to a specific device for an end user. While a plain SIP URI, such as firstname.lastname@example.org, is a URI that refers to a user, a GRUU is a URI that refers to a specific device.
Certain SIP scenarios ( REFER for example ) rely on the fact that a given URI will route to the specific device, but prior to the GRUU extension there was no mechanism to reliably do this.
The Registrar and Usrloc modules have been enhanced to support GRUU at the server side, so OpenSIPS will be able to generate public and temporary GRUUs to phones that support the GRUU extension, and it will also be able to properly route requests that target a GRUU.