Lots of ways. If the text box is bound to a numeric field
or if it has a numeric Format, Access will refuse to accept
a non-numeric entry. No Code required.
If you want to insist on the ten digits without +-. etc
characters that are part of some numbers, then set the text
box's or the table field's Validation Rule to:
Not Like "*[!0-9]*"
and you can specify any validation message you like.