Darren, for the sake of a succinct formula you might look to adapt the 2nd [edit:] approach from my prior post, eg:

G6:
=IF($B6="","",$E6*MAX(0,MIN($D6+1,DATE(YEAR(G$2),MONTH(G$2)+1,1))-MAX($C6,G$2))/DAY(DATE(YEAR(G$2),MONTH(G$2)+1,0)))
applied to matrix
in the above the following occurs:

i) calculate lesser of (end date +1) and (end of month + 1)

ii) calculate the greater of (start date) and (start of month)

iii) deduct ii) from i) with a MAX trap to ensure a value always >= 0

iv) calculate the number of days within the month (establish day no. of last day in month)

v) determine the apportionment % based on iii) / iv)

vi) calculate amount by multiplying Call Volume by v) %
as outlined, if you have the Analysis ToolPak activated (and/or are using XL2007+) the DATE based functions can be replaced by more succinct ATP alternatives, eg:

=IF($B6="","",$E6*MAX(0,MIN($D6+1,EDATE(G$2,1))-MAX($C6,G$2))/DAY(EOMONTH(G$2,0)))