Number format with SMS Clickatell provider
I just faced (and fixed) this problem three days ago for a client by patching the Clickatell extension to prepend the "1" where it doesn't already exist.

In short - someone already wrote this patch, but it only works if you don't have contacts you want to text outside the US and UK.  The patch is referenced on CRM-14160 - or just find it here.

If you're not a developer, but you feel comfortable installing an extension from a zip file, I just made a Github repo here with the patched extension.  That said - I don't want to post it as the "new" Clickatell extension, because it makes some country-specific assumptions.

You can see that I updated the CRM-14160 ticket with what I think is the ultimate solution - which is for CiviCRM to know the country code associated with each country.  Then the Clickatell extension could check for whether it's present or not - and if not, then prepend it.