Steve, we had one of these before (but as it kept connecting to Gibtel and Maroc Tel and cost us hundreds of euros, we cancelled it!). (Not in DV by the way, but in Bahia las Rocas where there are no phone lines).
It was a pay as you go for €50 a month (when it didn't decide to roam and was the best option at the time) but we took it to the local shop in Sabinillas (near Barclays) to top up. There is a way you can do it yourself, with a mobile and the code to input - which you can get from the vodafone shop or booklet.
Edited to say: Sorry - just remembered it was our contract dongle that managed to roam all on its own, not the monthly payg.
This message was last edited by Pitby on 22/02/2012.