It is roughly competitive with other global SIM card providers on its pricing for calls made in the UK, at 42c per minute, whether calling landlines inside or outside the UK.
However, it is free to receive calls in the UK, which is better than all the other providers.
The initial cost of buying a TravelSIM is $49.95, which includes $5 worth of call credit. They're available in all Australia Post outlets. TravelSIM cards come with an Estonian (+372) mobile number, so your contacts will need to call that at their cost -- but at least if you are travelling in the UK, you won't pay anything to receive the call.