Number as well as spacing requirements for phone numbers

We have had a few orders come through were the customer has placed a space in the required phone number field (and not their number) which has then been accepted by the system. We are finding this to be a problem and have suggested for the field to require at least some numbers as well as accepting a space.
The other suggest is that, customer have to put in numbers and any spacings will invalidate the field. I think most customers will know to then take any spacings out if this shows up as a problem for them.
I would like to know if anyone else has experienced this and would like this to be a change.