It looks like your data is in order by date and time right now?

If you're ok with more of a manual solution, you could : (Try to make sure you understand the logic behind these steps)

0. Make a backup of your workbook

1. Re-sort by type of call

2. Find where local calls start, highlight the amount column starting from that row down until the total showing at the bottom is just under 500 minutes (so that if you were to include one more row it would be over 500).

3. Write down this total

4. Type 0 [Enter], then press Control-D to fill down

5. Find how many minutes were left to reach 500 minutes: take 500 minutes minus the total you wrote down

6. Insert a row to break the next call into 2 as you described, put zero on the first row, and update the next row (will now be the first non-zero local call row) to subtract the # minutes left to reach 500 that you found in previous step.

7. If you need it back in the original order, re-sort by date and time.