I'm trying to send an information about number of unread messages in a voicebox to an UAC after its REGISTERing.
As i understand this can be solved via sending the MESSAGE method. Does openSIPS have a module that provides such function?
you can use the MI (Management Interface) to send SIP requests from external apps - see the t_uac_dlg MI command from TM module - http://www.opensips.org/html/docs/modules/1.6.x/tm.html#id229822.
Also check the php example on how to send a message: examples/web_im/send_im.php (in SVN and src tarballs)