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I don't believe that was the OP's requirement (but admit the post is not entirely clear). Receiving a voicemail is free, retrieving a voicemail isn't always free.

The OP didn't say what network he was on, however if you are on O2 as per your example, you can retrieve them for free as well if you have an internet connection, which the OP does, and the same applies to at least one other network (and possibly iPhones in general).

Charges only come into play if you explicitly make an outgoing call from your handset to your voicemail provider, but that's no different to making any other sort of call - you have to do it yourself, if you do nothing, you will not be charged. Retrieving depends on your tariff and some people get charged to retrieve voicemail even in the UK, but receiving is always free.