If you are really hungry for wifi/GSM, there are phones out there that support quad-band GSM, plus WiFi, with SIP. So you can do VoIP when in range of WiFi. These phones are mostly being OEM'd to VoIP companies at this point, but they are out there. Here is a sample...
FT77dm Wi-Fi/GSM S(IP) HANDSET
FREEDOM OF MOVEMENT IS A FUNDAMENTAL EXPECTATION
Introduction
The Global standards Wi-Fi broadband and GSM solutions that leading operators and
carriers are deploying around the globe!
The FT77dm blazingly advanced wireless/mobile tele- & datacom solution keeps you
connected everywhere anytime, @ home, @ work, on the move, everywhere in between
Wi-Fi and GSM networks.
Dual mode GSM/Wi-Fi IP phone adopted for the leastcost
tariff, highest bandwidth and most reachable
services.
USB interconnection to laptop/PC for high-rate data
connectivity in enterprise network.
Bluetooth wireless modem to PDA or Notebook PC for
the internet access either via WLAN, GPRS or Dial-up
Networking.
Bluetooth voice connection with FT77dm.
FT77dm PAN Application
Who will buy
Frequent traveler
Mid-top management
SOHO, freelancer
3C gadget early adoptersr
Mobile distributor
ITSP
Wireless operators
IP-PBX supplier
Bluetooth specialist
SI
Least cost for corporate voice communication
Extension on the go (local or remote) -> no more missing calls
Seamless communication (FT77dm) -> cross WLAN & GSM
Landline/DECT replacement
Easy handling -> programmable Prefix & speed dial
Simultaneous data access (FT77dm)
Who will buy
Frequent traveler
Mid-top management
SOHO, freelancers
3C gadget early adopters
Sell through
Mobile distributor
ITSP
Wireless Operators (brand name requested)
IP-PBX distributors
SI
Specification
Bluetooth
Version 1.1 Class II
HSP, HFP, SPP, DUN, CTP profile
GSM/GPRS
Quad band GSM 850/900/DCS 1800/PCS 1900
2 SIM slots
SMS messaging
Wi-Fi
IEEE 802.11b
11Mbps with Dynamic Bandwidth Management
40 bit and 128 bit WEP data encryption
802.1x authentication
QoS support TOS / DiffServ
VoIP
GSM and G711, 723.1, 726 and 729ab
Outbound Proxy for NAT Traversal
Adaptive jitter buffer and echo cancellation
G.168 echo cancellation
Silence suppression, Voice Activity Detection (VAD)
Comfort Noise Generation (CNG)
DTMF detection and relay
FT77dm Functions & Features
Specification
Network and Protocols
SNMP manager
DHCP or Static configuration option
Alternate TFTP support
SIP (RFC 3261) version 2
SDP (RFC2327)
RTP (RFC1889)
RTCP (RFC1890)
Support outbound proxy for NAT traversal
Site survey, Trace route (hidden feature), Seamless-secure
roaming and VLAN support
STUN server supported
Calling Features
Caller ID display
Call waiting
Call forward
Call transfer
Pre-dialing before sending
Call mute
Redial
Call hold/ resume
Call park
FT77dm Functions & Features
Specification
User Interface
Color STN Display 96RGB x64 Pixels with LED backlite
Keys : Power , Volume, Menu Selection, Send/End, 12 keys dialer pad
Selection of Menu, user profiles, languages and ring types by soft keys
Hotkey for keypad lock
Hotkey for voice-mail access, call logs
Ten configurable speed dial
Idle / call state-based soft-keys
Local phone book
Time / date display
Keypad lock / vibration icon indicators
RF Level and battery gas gauge indication
Polyphonic ringer, vibrator and visual display alerts
Easily Switch VoIP/GSM voice; WiFi/GSM; SIM 1 / SIM 2 by one key
Pocket PC 2002/2003; Windows based WIM management and network
configuration utility
FT77dm Functions & Features
General
Intel PXA255 100-400 MHz, scalable
16 MB Flash, 8 MB DRAM
USB 2.0 (Slave)
1300 mAH Li-Ion rechargeable battery
98 x 64 CSTN LCD with LED backlit
12-key dialer pad with LED backlit
Function keys implemented
Speaker/Microphone/Headset Jack/Charging Jack
Downloadable WMA Polyphonic ring tones
Vibrator for call alert
72 hours standby and 4 hours talk time
Dimensions (H x W x D); 100mm x 60mm x 22mm
Weight (with standard battery): 128 g