numbers don't sort 'numerically' if they really are text. Leading 0's for example will ruin a numerical sort.
So, make a field
GetAsNumber ( <<your field here>>) // number type result, of course
and sort on that. the field need not be on the layout to be used in the sort.
look at various records to verify you have the correct number. If it's working well, don't make a calculation field (stored or unstored), but an auto-enter number (by calculation) that will update for each new record.
Thanks for the reply, unfortunately folks here are intent on using Filemaker in the wrong way and I cannot amend either the programming or their behavior.