That number should be charged the same as a call to the mobile operator Three, which would be the fm6 tariff band from landline:
Online CodeLook
However, as you're finding, the mobile operators (especialiy for payg) don't have to follow those bands. Still seem excessive and wrong though, maybe they were looking up a +77 international call?
Sims: Telcel MX, T-Mobile US, Virgin Mobile UK, Orange UK, Knowroaming
Voip: Localphone UK, Anveo US, Google Voice