kamailio.cfg: add colon before the port for voicemail server

(cherry picked from commit 0e3f1ff9507a578d95d02d615d333ef24fe42d87)

etc/kamailio.cfg

@@ -884,7 +884,7 @@ route[TOVOICEMAIL] {
 		return;
 
 	$ru = "sip:" + $avp(oexten) + "@" + $sel(cfg_get.voicemail.srv_ip)
-				+ $sel(cfg_get.voicemail.srv_port);
+				+ ":" + $sel(cfg_get.voicemail.srv_port);
 	route(RELAY);
 	exit;
 #!endif