4 call in 8 hours give you 0.5 calls in Busy hour. The BHCA of 0.5 multiplied by 360 seconds of Call hold time gives you 0.05 erlang traffic per subscriber.
as 10% traffic goes through the toll, it means 0.005 erlang per sub in busy hour is the traffic requirement.
1 MHz i am assuming half E1 that gives 16 servers. (assuming PCM with 64 kbps time slot). 16 timeslots will give you almost 5 erlangs (4.8 er). 4.8 divided by 0.005 gives you around 960 subscribers.