Virgin media are perfectly allowed to implement a call limit especially for new customers in order to both protect virgin media and prevent customers from running up huge phone bills.
If the bill is in the region of £100 they need to take a good hard look at the call package they are on, when they are making calls and who to as frankly a landline bill of that a month is shocking (we spend no more than £10 a month over our minimum rental on virgin media on calls)
If they are calling landlines, on the current package they are only free at weekends for up to an hour and then they will have to redial. Calls at all other times are chargable however they can get free evening and weekend calls to landlines by upgrading to package L or free calls to landlines anytime on XL.
Calling mobiles, adding the call mobile add on will reduce a little however my personal recommendation would be to pick up a cheap phone on vodafone pay as you go and activate vodafone stop the clock and free weekends. They will then get evening calls up to an hour for 30p with free calls to vodafone and landlines at weekends and an hour of calls to other networks at weekends (after which we are back to 30p for an hour)
Calling 0870 or similar numbers, check out
SAYNOTO0870.COM - Non-Geographical Alternative Telephone Numbers and use the alternative numbers.
Calling premium rate.... Well just STOP IT!!!