Wireless telecom system (cellular with Erlang and spectral efficiency)
Check WCY Lee's book these are old AMPS calculation methods for spectral efficiency, there are some graphical methods to estimate efficiency using hexagonal geometries.

Page 186 - Mobile Communications Design Fundamentals (By William C. Y. Lee)

Also to calculate offerred traffic you have to convert call attempts/hr to attempts/second, thus

\$  \LARGE A_o= \frac{\frac{1.5 calls}{hr}}{\frac{3600s}{hr}} \times 120s \$

\$ \Large A_o= 0.050  \text{ erlangs or 50 } \frac{mErlangs}{sub}\$

Here is the link for the book:

Mobile Communications Design Fundamentals W CY Lee