I'll be very curious to learn what the various US international carriers and callback services will charge to call the +44937.... range of numbers.

I have a hunch Barry @ CBW will have a very attractive callback solution for +44-7937.

That will probably depend on them having one rate to dial uk mobiles or not. If they have one rate, then they will probably include it there.

Voiptalk (uk voip provider) charges 24 pence/min to connect to 07937 numbers.

Do you mean "euroroaming zone" or "zone for outgoing calls from the home country"?
You're basically right about second option, I think. As you said, the difference in pricing (if any) between UK and the crown dependencies with the +44 numbering may occur for VoIP providers. In Poland, so far I've only one VoIP provider which charges moreless twice as much calls to +4476, +4479 and 3UK than to other +44 mobiles. No mobile or landline provider distinguishes those territories.
However, with roaming the things are different. So far, 3 out of 4 operators here declared their euroroaming tariffs. Plus and Play don't mention about Channel Is/IoM so they probably consider them as parts of UK. However, Era in its roaming tariffs declared those territories explicitly as non-EU (both as origin and destination of calls).

Based on the [United Mobile will deliver a combination of Truphone, Jajah and Skype on a "One SIM card Service"] statement and the new Jersey card that they said will have UMTS data service in 4q07, I wonder if they will have the option of VOIP calls over UMTS and if this would be cheaper than either a standard GSM callback on United Mobile or using a CBW solution?

<i>However, with roaming the things are different. So far, 3 out of 4 operators here declared their euroroaming tariffs. Plus and Play don't mention about Channel Is/IoM so they probably consider them as parts of UK. However, Era in its roaming tariffs declared those territories explicitly as non-EU (both as origin and destination of calls).</i>

Umm, yes! The Isle of Man and the Channel Islands are not in the UK and not in the EU . . .

And how about Gibraltar then? http://en.wikipedia.org/wiki/Gibraltar#Politics says:
Gibraltar is a part of the European Union, having joined under the British Treaty of Accession (1973), with exemption from some areas such as the Customs Union and Common Agricultural Policy.

Does this mean that euroroaming tariffs apply to Gibraltar?

Yes, Gibraltar is surely inside eurotariff

O2 Czech Eurotariff Offer Including list of countries
 

O2 Eurotariff

One rate for calling within EU countries.

We are offering a new tariff for favorably-priced calling in all 27 EU countries.

Calling between EU countries (including the Czech Republic) costs 13.95 CZK/min (16.60 incl. VAT). For incoming calls you pay 6.85 CZK/min (8.15 incl. VAT).

Activation

The new O2 Eurotariff can be activated from July 29, 2007 in any of the following ways:

* Corporate line 800 111 777 (*77 from your mobile)
* Business line 800 203 203 (*52 from your mobile)
* By contacting one of our business representatives
* Entrepreneurs can send an SMS with the keyword EUROTARIF to the phone number 999 111

Activation takes place one month after the customer has submitted their request for the service. After the service is launched on August 29, 2007, all customers will be able to activate O2 Eurotariff together with the O2 Roaming service.

O2 Eurotariff cannot be combined with other special roaming offers, including individual roaming conditions stated in the Framework Contract. Once O2 Eurotariff is activated, all of these special offers, and their respective benefits, are deactivated.

Prices
Zone Outgoing calls within zone
and to the Czech Republic Incoming calls SMS MMS
EU 13.95 / 16.60 6.85 / 8.15 10 / 11.90 7.80 / 9.28
Rest of Europe 35 / 41.65 20 / 23.80 10 / 11.90 7.80 / 9.28
Other countries 55 / 65.45 45 / 53.55 10 / 11.90 7.80 / 9.28

EU – Austria, Azores (Portugal), Belgium, Bulgaria, Canary Islands (Spain), Cyprus, Denmark, Estonia, Finland, France, French Guinea (France), Germany, Gibraltar (Great Britain), Great Britain, Greece, Guadeloupe (France), Hungary, Ireland, Italy, Latvia, Lithuania, Luxembourg, Madeira (Portugal), Malta, Martinique (France), the Netherlands, Poland, Portugal, Réunion (France), Romania, San Marino, Slovakia, Slovenia, Spain, Sweden, Vatican (Italy)

Rest of Europe – Albania, Andorra, Belarus, Bosnia-Herzegovina, Croatia, Faeroe Islands, Guernsey, Iceland, Isle of Man, Jersey, Kosovo, Liechtenstein, Macedonia, Moldavia, Monaco, Norway, Serbia and Montenegro, Switzerland, Turkey, the Ukraine

Other countries – All remaining countries (including Russia)

Vodafone Czech Eurotariff
 

Vodafone Czech will include in Eurotariff countries EU plus Croatia and Switzerland.