Hi Leeg and welcome to the forum!
In answer to your questions here and in your other posts. You should speak to the public sector accounts teams of the major mobile operators, e.g. O2, T-Mobile, Vodafone, 3, etc that have good 3G data coverage in your area.
If you're concerned about voice calls you should be able to contract with there operators for data only contracts thus avoiding voice call charges. Some of these contracts do allow a "pay as you go" voice option that allow the student to use the phones for talking also. Subject to your requirements you might also want to consider other technologies such as Sony PSP which have excellent web browsing capability over wireless and are significantly cheaper than mobile phone based devices. You may also find that many of your students already own one!
Be aware of some of the restrictions that have been put on mobile internet devices to limit access to various internet sites. To prevent access to adult material the filters are still quite primitive and are causing a lot of upset amongst students who can't access their favourite non-adult sites. Annoying if they or their parents are paying for it.
There are many devices available and you should decide what you want them for and test a few models with the help and input of your students.
Be aware that if you're using all the devices in school at the same time it is likely that the 3G connections will not keep up as cell antenna's have limited capacity so your devices will need to use the school wi-fi which can also be a challenge and may require a substantial upgrade in infrastructure and bandwidth.
Last but certainly not least is determining how these devices will be embedded within teaching practice and what support will be provided to your teachers.
Good luck and let us know how you get on.
