It's worth putting some constraints and background facts up first:
- In the US (where quite a high % of enterprise FMC vendors are based) there are no differences between fixed and mobile numbers
- Elsewhere, especially Europe, mobile numbers are visibly different to fixed numbers, eg 07xxx
- Most people prefer using mobile phones for many calls, especially outgoing calls where the phonebook is convenient. This varies a lot by function, country, age and individual preference, however. Call centre operators and receptionists don't use mobile devices for the most part, for example.
- A large and increasing proportion of users use SMS for business applications. This comes as a shock to many, especially those from the US. It's typically used for 'casual' messages like 'In taxi. There in 5 mins' or 'Delays on tube, will call you when nearby' or 'Your meeting this afternoon has moved from Booth#1321 to Booth#432'. While it's not mission-critical as such, it has high convenience factor, lots of user inertia, and is much more likely to be received in many cases than email (which of your clients/colleagues have BlackBerries? are you sure?) or IM or even voicemail. But most people won't SMS to a fixed number as fixed-SMS functionality isn't widely understood or 'expected'. In fact many phones' SMS client software won't even allow you to text to a fixed number.
- People almost never change the numbers in their handset's phonebook unless they absolutely have to.
- Some calls are made to people, others are made to places. I don't want/expect a mobile number for my pizza, my doctor's surgery, or the customer support & billing department at my mobile operator. Apart from anything else, in some markets it can be seen as 'rude' to expect your customers to dial into a mobile number from a fixed-line when it's not necessary, as it costs much more owing to larger termination charges.
- I think that not all countries have 'inclusive' cross-network minutes for mobile-to-mobile calls. Again, this increases the costs for your clients or suppliers to do business with you, rather than (say) an 0800 number or a fixed phone. Obviously if you're calling a field engineer or a salesperson or a plumber you expect them to be on their mobile - but many people (and their employers picking up the bill) will resent paying a 10x premium to call an 07xxx number for someone they know is at their desk or perhaps on shop floor.
- Many workers will have a separate mobile phone for personal use anyway. They may well get a bucket of cheap minutes & not worry about getting reimbursed by their employer. If the user experience & convenience is better, they'll use that device for preference unless there's a harsh proscriptive policy 'use a personal phone & you're fired'.
All this means there's no easy answer. Leaving aside the oddity of the US numbering scheme, all 3 alternatives for FMC numbering have both positives & negatives:
- One fixed number - allows the enterprise to 'own' the number, routes via the PBX with all its benefits... but has problems handling SMS and the fact that it's almost impossible to 'hide' the handset's real underlying mobile number - which of course will then be stored in their contacts' mobile phonebooks.
- One mobile number - works quite well for certain roles where employees can conduct all their calls from a mobile. Doesn't work so well in contexts where existing PBXs must be integrated, or where there are reasons for wanting geographic numbers for sites or departments rather than individuals. Issues with inbound call costs, plus issues with how easy number portability is to manage in reality during churn. More difficult to do least-cost routing for enterprises to minimise internation call charges.
- Mobile + fixed number - Solves the above problems, but possibly higher costs to administer, and makes 'reachability' more difficult. Made even trickier if the user also has a second mobile number for personal use or an email device.
Bottom line: there's no single right answer. It will vary according to geographic market, function(s) of the users concerned - and how proscriptive/flexible the employer is. What I think is the wrong answer is for any vendor to focus exclusively on any one of the above options, unless they have a very specific target niche in mind (eg small businesses in Scandinavia, perhaps).
As an interesting example, it's worth noting that Vodafone's enterprise FMC team have both fixed & mobile numbers on their business cards.