It is a staggering statistic. According to John Wright, general manager of mobility at BT Retail, four out of ten mobile calls are made from inside offices. Forget the cost – the modern day business executive rates convenience above everything; there may be cheaper communications methods available, but they cannot compete with the ease and familiarity of the mobile phone.

For the businesses that, in many cases, pick up the tab for those calls, it is a large and growing problem – but one that could shortly be resolved. Network operators and infrastructure providers are poised to launch a raft of new products and services that promise seamless communication unrestricted by location.

In-building wireless (WiFi) networks that can support voice over Internet protocol (VoIP) to the outdoor cellular network promise to preserve all the benefits of mobile technology, while slashing call charges. Globetrotting executives visiting offices abroad, for example, can still use their mobile phone but will no longer need to 'roam' onto expensive foreign networks. Other benefits include better integration with enterprise phone systems, allowing improved transferring, parking, monitoring and filtering of calls. Users will also be contactable on the same phone number regardless of device or location.

The general release date for many of these new services is 2006, but some functions are becoming available sooner as vendors such as Avaya and Cisco put their software onto smartphones – programmable mobiles with advanced operating systems. However, not all of these devices can provide seamless roaming between networks, which proponents suggest is vital if phones are to use the cheapest available network.

"It needs to be simple and easy. People are too busy for that extra keystroke," says Wright of BT. "The vision we have is convergence whereby the [calling] experience is not degraded whether the user is linked locally, at the office, at home, or out and about."

He envisages that phones will also connect via emerging wireless networks, such as WiMax or high speed download packet access (HSPDA), but warns that as access technologies multiply, switching between them manually will become problematic.

Communications fusion

Proving that seamless handover could be achieved was the key success of Fusion, BT's long-awaited converged phone service. "This is a watershed – separate fixed and mobile telephony services are no longer discrete but are intertwined," gushed UK market watchers Ovum at the launch in June.

BT may have achieved a world-first with a regular mobile phone (in this case, a Motorola V560) that can use both a landline and mobile networks. However, Fusion is aimed at consumers and small businesses. The use of Bluetooth, a short-range radio technology, to connect the phone to the in-house hub, and unlicensed mobile access (UMA), a technology for handing the call from the home network to the public one, means it is not suitable for large enterprises. Businesses need such a device to make use of existing WiFi networks and to integrate better with their private branch exchanges (PBX). However, BT is working on a WiFi version to replace Bluetooth. This will also be able to make VoIP calls in BT's Openzone wireless hotspots.

Taking advantage of the price benefits of VoIP beyond office wireless local area networks (WLANs) is a key selling point of such systems. BT's Wright suggests that mobile calls might typically take up 10% of the volume of all calls on an enterprise phone bill, but 50% of the cost. "Fusion is looking to bring together the best of the fixed world – in quality of service, cost and control – and add mobility."

However, enterprise-class Fusion will not be available until mid-2006, partly due to handset delays. Few affordable devices capable of both WiFi and GSM calls are available, but many are in development. Cisco, for example, is working with both Nokia and Motorola on 'dual-mode' phones, which will enable continuous calling across cellular networks and WLANs. Both are due for release in 2006 and will tie into existing Cisco equipment, tackling the tricky issues of seamless mid-call network roaming and device authentication.

Nokia has also developed a dual-mode WiFi and cellular handset with networking equipment provider Avaya, which can detect and use WiFi when available. But while supporting the fundamentals of converged communications – one phone number, one mailbox, cheaper in-office calls – it will not be able to sustain calls moving between WiFi and cellular networks.

Fine form

But then not everyone agrees with BT that seamless handover is so crucial. "How often do you walk from the office to the car while on the phone?" asks Jerry Caron, principle analyst at research firm Current Analysis. "Is it that big a deal to say 'I'll call you right back'?"

The final form factor and functionality of dual-mode handsets is more important, says Caron. "Those devices really need to be multi-mode – the exact same people who benefit from PDAs, smartphones or BlackBerrys today are the people who will benefit from dual mode devices. So whatever beast comes out cannot be voice only, it has to be able to support business applications."

Avaya and Research in Motion have already joined forces to extend applications to the latter's BlackBerry device using the session initiation protocol (SIP), which can make any device communicate with any other. They are starting with VoIP but plan to develop business applications for the handheld general packet radio service (GPRS) emailer. But GPRS will not provide enough bandwidth for applications such as videoconferencing, which means for truly seamless convergence of applications and services from wired to wireless, 3G networks will have to come into play.

Here again, device design is key. "Most high-end phones are becoming 3G anyway and to do voice over WLAN you need a high-end phone because you need a fast processor and a smart operating system," says Dean Bubley, a wireless VoIP expert with Disruptive Analysis. Battery life is also an issue, he says. "A phone is very tightly engineered to a power budget. WiFi and VoIP applications make more demands on processor requirements and on the battery. Running [free consumer VoIP service] Skype on a PDA requires at least a 400mhz processor but even Symbian smartphones today rarely have more than 200mhz."

Changing relationships

The technical challenges of passing voice calls between 3G and WiFi are more than enough to keep developers and engineers busy. But there are also some tricky business issues to overcome. VoIP calls are not metered by the minute, undermining most operators' current business models. Even diverting in-office calls from their networks could seriously dent their revenues.

Vodafone has not taken kindly to the threat. Its German unit has threatened to block some VoIP calls to its network and has introduced aggressive new international roaming tariffs with its 'Passport' service – all designed to encourage customers to ditch landlines altogether and go completely mobile.

Analysts are certainly divided on the size of the converged opportunity: Infonetics Research's latest survey into voice over wireless LAN (VoWLAN) found that 36% of respondents are planning implementations, up from 13% in 2003. Specialist wireless advisory group Farpoint estimates that half of all mobile phones will have WiFi functionality by 2010, but Bubley of Disruptive Analysis predicts SIP-based dual-mode handsets will make up only 5% of the market by 2009, with 5.8 million sales worldwide.

That caution may well reflect the potential complexities of managing and integrating so many products and services from different vendors – described by one IT manager as "finger pointing hell". Few operators have even worked out their billing plans and business models for this converged world.

While the business goals of ubiquitous, real-time communications are clear, the technology that will enable it remains a topic of fierce debate.