We are already using Asterisk for our communication so the new mobile handsets are required to work well with it. Communication should be secure and ideally the handset would also be a mobile (GSM) phone that could be carried along when leaving the office.
Our choice was the Nokia E63. It supports quad-band GSM, UMTS, SIP and WLAN IEEE 802.11b/g including WEP, 802.1X, WPA and WPA2. Combining GSM and SIP + WLAN works out great. In the office we directly receive landline and VoIP calls to our mobile. When we are away Asterisk detects the missing phone and either routes calls to the mobile number or sends callers to the mailbox.
The solution is affordable, based on open standards, secure and very user friendly.