if you know the length of the number and the identifier is Number:, then you can give this a try:
local numbervar phone;
phone := instr({tbl_name.desc}, "Number:")
if phone > 0 then
{tbl_name.desc} := left({tbl_name.desc}, phone-1) + mid({tbl_name.desc}, phone + 6 +lengthOfNumber
I am not sure about operations on the field itself, I usually place it into a variable, but if has worked, and I can't see why it wouldn't, all the better.
If there is more than 1 number in the string, you can loop. Check the help on how to do that. It's not hard, but I don't remember how offhand.