If you are still on the same tariff you took out 10 months ago then your data is still unlimited and will remain so as long as you remain on the tariff.
The data stops being unlimited as soon as you change anything, ie renew the contract, change the tariff, or change the data bolt on. Unfortunately for you changing a blackberry necessitates changing the bolt on to a blackberry- specific one, and this will come with the 500 mb data limit. As long as your current plan runs unmolested nothing would change, so I would either consider another type of phone that will work with your current account settings (nothing to do with the sim, everything to do with the account setup) or, look into your data usage, is 500mb really not enough?
The main catch with Three, for me, would be 'will it work?' I notice there doesn't seem to be an option to add BB services to the deal, even when you add it to the basket, but it does stick on a nice £8.99 per month geek squad charge. It's a strange one.
Have you looked at o2's direct BB offer which strikes me as better than that? It's £25, but you get 500 mins, unltd SMS, 500mb web browsing plus BB services and wifi all included?
The o2 plan is called Blackberry bonus because tyou get the extra minutes. Is the other one you mention on Tesco, rather than o2? AFAIK that one only offers 250 mins, it entirely depends on what you wantand if youre a light user, like me, then it should be fine.
only you can decide whats best for you. I would just say look at what your getting as well as what your paying and also check what is and isnt chargeable as networks differ
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