If you are searching for business class Mobile Phone with WiFi and Sip Client option ? Best one is Nokia N61i. Take a Look here:
Why is the NOKIA E61i the ultimate mobile phone ? Just look at its spec !
As NOKIA E61 + 2M Camera and Videocam
Symbian OS v9.1
Music Player
VOIP SIP softphone built in
Qwerty keyboard
Email
S60 3rd edition
Quad band + WiFi WLAN
USB
MIDP 2.0
CLDC 1.1
IPSEC VPN
WiFi hotspot finder !
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