Featuring Android 2.3, or "Gingerbread", the Desire S is the first HTC Android Smartphone to come with a SIP VoIP client built in, enabling you to place calls over an Internet connection to a SIP provider. If your company telephone system supports the SIP protocol, you can also set up your Desire S as a wireless extension of the phone system when in the office, or even when out of the office with some configuration of the phone system.

NOTE - the VoIP client currently supports voice only, you will need to install an application such as Skype or Fring to do video calling.

Siax is a fully functional VoIP client for the iPhone, providing support for both SIP and IAX protocols, enabling you to make intra-office calls remotely via either 3G or WiFi connections directly from your iPhone. Interfacing with your existing contacts list and providing support for multiple VoIP accounts, Siax is very intuitive to use and easy to configure. Note, however, that Siax is not free, licenses costing in the region of £6, and also requires that your iPhone be jailbroken.

The Nokia E75 has a SIP-based Voice over IP (VoIP) client built-in, meaning that the handset can make and receive calls over the Internet provided that the SIP PBX can be configured to support the device.

In this post I shall be using SIPtrix, the Asterisk-based Linux PBX server. There are a number of pre-requisites that need to be satisfied on the server:

The SIPtrix server must have a ‘real world’, ‘routable’, ‘public’, or ‘Internet-facing’ IP address that is available from the Internet. Without this, no remote connectivity will be possible.

Before the Nokia E series handsets can be used as remote extensions, there are a number of pre-requisites that need to be satisfied on the server:

The SIPtrix server must have a ‘real world’, ‘routable’, ‘public’, or ‘Internet-facing’ IP address that is available from the Internet. Without this, no remote connectivity will be possible.

The Unified Messaging server role of Exchange 2007 offers the ability of integrating your IP PBX with Exchange, so that voicemail messages are delivered directly to the user’s Exchange Inbox.

The BlackBerry Mobile Voice System (MVS) is a server-based product designed to sit between your existing BES and your IP-capable PBX, effectively turning your BlackBerry handheld device into a remote extension of the corporate telephone system: if you are out of the office, or just not at your desk, incoming calls to your desk DDI can be forwarded automatically to your mobile; and similarly any calls you make from your handheld can be routed out through the remote PBX so that calls appear as having originated from your office (and can be billed accordingly using the PBX’s least-cost routing ca