adding new data to a table with sequential data

[Thor Tryggvason]

Hi
I am trying to enter data into a table that has semi-sequential data in it.

Example of data in table customers:
phone ; Name ; number
292-123-4567; John Doe ; "null"
292-123-4567; Granny Smith ; 1
292-123-4567; Jane Doe ; 2
345-567-1234; Peter Doe ; "null"
456-567-2345; Pete Doe ; "null"

In other words, if there are more than one name registered on any phone number, the sequence number should increase by 1 on insert of new row.

Then I have a table import with customer data containing e.g.:
292-123-4567; Grandpa Smith
678-123-4567; Jane Doe

The first row should be inserted and get a sequence number "3"
The second row should only be inserted into customers with sequence number "null".
There are hundreds of rows that need to be inserted every week.

Any suggestions on how to do this? I am totally lost.

Regards,
T