O2 Blackberry Hansets on Vodafone BES

We have been running a Blackberry Enterprise Server (BES) connected to an Exchange 2003 server for over a year now. We have been asked by our sister company to supply them with a Blackberry service. The problem was that they only recently negotiated a very good O2 contract for all users and we use Vodafone here. We bought the BES server and all handsets from Vodafone.
We did think that we would have to replace, or buy out, all the contracts of the people who want Blackberry with Vodafone handsets and SIMS. This would of been a nightmare. The other option was to build a new BES with software supplied to us from O2. Also a nightmare.
So we tested an O2 SIM through an O2 Blackberry 8800 Handset which is serviced via our Vodafone BES. It was found to work perfectly. This way all users can keep thier contracts with O2 and all we do is pump e-mails to them.