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.

Configuring an internal extension

Internet Calling settings are located under Settings --> Call on the desire S menu:

Click on Accounts to create a SIP account:

Click on the option to Add Account:

Here you will need to enter your username (often your extension number), password and the internal IP address of the SIP server.
Your administrator will probably want to create a separate extension from your deskphone if you have one and have you register using this extension number.
Additional options are also available if you need to select TCP as opposed to UDP communications, SIP port number and proxy server settings can also be defined if required:

Once all settings have been entered, press the menu button and Save the account.

The new account will be listed. To poll the server for incoming calls, tick the option to Receive Incoming Calls. The registration status of the new account will be displayed:

Back within the Internet Call settings, you can specify whether you want to always use Internet calling, or whether you want to be prompted which carrier to use each time you make a call:

If you select the option to choose, when you place a call within the Phone dialer, you will be prompted which service to use:

And you will now be able to dial internal extensions directly from your Desire S, not requiring that you be at your desk, but not paying for the cost of a mobile call:

Configuring an external extension

In order to register the Desire S as an extension of the phone system when not in the office, you could configure a VPN connection on the Desire S and use the same settings as those when in the office, but VPN capabilities of the Android platform are still limited.
Alternately, you could connect over the Internet.
The SIP 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.

In this example I am using the open source Asterisk SIP PBX.

If the SIP server is behind a firewall, then UDP port 5060 should be allowed through to the server from the Internet, for SIP signalling traffic. The UDP port range 10000 to 20000 must also be allowed through for RTP traffic (the ‘media’ of the telephone conversation).

The SIP server must also be ‘made aware’ of its own external IP address as well as the local network on which it sits:
Log into the admin interface for the server via the web interface and browse to Tools → Config Edit. Locate the ‘sip_nat.conf’ file and add the following lines:

nat=yes
externip=81.138.13.178
localnet=10.0.0.0/255.255.255.0

(you will need to substitute the above IP details for the configuration of your own external and internal network addressing scheme).

Once these pre-requisites have been satisfied, you must now create an extension on the system for the phone to register as.

Within the admin interface, add an extension in the correct number range (1xx would be suitable for remote extensions to identify them as such and separate them from the 2xx internal extensions).
Set the Outbound CID number to match that of the person’s desk extension.
Set the secret to password or something suitable.
Set the NAT field to ‘yes’
Disable voicemail.

(Optional)
Create a ring group and add the person’s desk extension and the Desire S extension as members. Set the Inbound route for the person’s DDI to point to the ring group.
Set the ring strategy to ring all.

Submit and Apply the changes.

You can now configure the SIP account on the phone, using the server's external IP address.