Can anyone please help me solve this frustrating problem!

I have 5 columns of info about my phone calls (the standard stuff: date,
start time, no called, duration, cost). Column E is the 'amount' column,
and I want to total all the amounts for calls which (in column 3) START with
a 7. In other words, I'm trying to find out how much was spent on all calls
to mobile phones (all leading zeros are removed).

Its easy to find totals for calls to particular numbers, but I can't figure
out how to total them all. The only criteria I can think of is that they do
all begin with a 7.

I've tried using 7* in both sumif and dsum scenarios but it doesn't work.
How can it be done? Thanks
Jeff