You can reach the busy hour figure from monthly figures by applying certain assumptions. Be careful though: they do not always hold true, and can vary across different industries.
If you have monthly traffic figures in minutes, then you can convert that to Erlangs by dividing by 60. Then divide that figure by the number of working days in a month to get the daily traffic figure in Erlangs.
Apply a factor to convert the daily traffic figure to the busy hour traffic figure. That’s the tricky bit, but a factor of 17% is often used (ie. 17% of the daily traffic is offered during the busy hour.