Interesting. A VLOOKUP based solution might work but I'm not sure I
altogether understand the problem:

If there are a lot more entries in Sheet2 than there are in Sheet1, can
multiple entries on Sheet 2 match a single entry in Sheet 1? If so, does the
spreadsheet need to count how many entries on Sheet 2 match the same entry on
Sheet 1 and divide up the cost among them?

Or do some of the entries on Sheet 2 not match any entry on Sheet 1? If so,
whats the rule to determine that theres no match? Cant find any entry in
Sheet 1 within 4 minutes of the time on Sheet 2?

Can calls be closer together than the time skew so that the following
situation is possible?

SHEET 1
Date - Time - Number Called Duration Cost
01/02/10 09:05:21 01234123456 02:16 0.50
01/02/10 09:05:24 01234123456 02:16 0.50

SHEET 2
Date - Time Number Called - Duration Extension
01/02/10 09:05:23 01234123456 02:15 1234

If so, what would be the rule to decide whether the entry on sheet 2 matches
the first or second entry on sheet 1?