Most PBX players have some level of relationship with Microsoft, and many of these ties have been strengthened in recent months. As dual-mode phones that operate on both cellular and 802.11 frequencies begin to hit the market, mobility is becoming an ever more important part of IP PBX player strategies. Line shipments of IP PBX systems will increase from 19.1 mln in 2006 to 37.8 mln in 2010, In-Stat says.