Rules for copying down

[RLevkoy]

I am trying to understand the Excel rules for changing cell contents when copying down by dragging the cell handle.  They do not appear to be consistent.  For example:

1. If I have a phone number in a cell and copy it down by dragging the cell handle, sometimes the area code increases by one digit and sometimes the last four digits increase by one digit.  Excel used to just copy the same phone number down, which is what I wanted.
2. I have a column which is formatted as text but I use it to record invoice numbers.  I used to be able to pull down the cell handle and have it copy the invoice number for a group of items on the same invoice.  Lately Excel has been increasing the invoice number by one digit in each cell.

I cannot tell what the rhyme or reason is and am not aware of a setting that controls this.

[#2]

@RLevkoy Any text ending with a digit will increase by 1 when you just drag it down by the fill handle. So, "Text1" becomes "Text2", "Text3" etc, Pressing Alt while dragging cancels the automatic incrementing of the number. So you will get "Text1", "Text1", "Text1" etc,