Is Erlang B formula still applicable to dimension the no. of channels for ISP traffic which has unique charateristics like longer duration per call etc.
The erlangB formula can be applicable here but I believe you should use the EngsetB formula for data traffic.
Indeed, call holding times are longer and busy hours are longer which means you will need less capacity to carry your traffic in the busy hour(s). I use about 450k minutes per E1 for calculation for ISP traffic.