Formatting phone numbers
For display purposes, you might be able to use the custom format string "00000000000" (without the quote characters):
(Once I did that, Excel thought I was using a Polish format. Upon closing and reopening the Format Cells dialog, I saw the following.)
You can even include additional characters, such as spaces (normal or thin) and/or + - ( ) if those are appropriate:
and other characters can also be included by surrounding them with quote characters within the format string.
But your sample data includes both 11-digit (you say) numbers and 12-digit numbers (as is obvious from the scientific notation values). That could be a complication. If both require a leading zero, you might use a custom format like "[>99999999999]0000000000000;00000000000" (again, without the quote characters), which effectively says "if the number has more than 11 digits (ignoring leading zeros), format it to use (at least) 13 digits, adding a leading zero if needed; otherwise format it to use 12 digits, adding a leading zero if needed".
If none of those work for you, please be more specific on the possible layouts of the phone numbers in your data, and your formatting requirements. And mention which version of Excel you are using, on which platform.
As for the "analysis between the different data sets", that makes it appear as though you "need" these numeric values as text. If so, you might import the data as Text (described in the earlier reply) or consider reconversion with a formula using the TEXT function.
