Number validation on user entering mob number in profile

Hi,

I can't work out how to create a custom number validation function to hook into the point where users enter their mobile numbers for the first time.

I have the following modules installed:
Clickatell
SMS International
SMS User
Tracking
Validation

When I set the number validation admin setting to use 'only the main validation function' and then use the test feature, I can't get any of the functions in SMS international or Validation modules to fire.

The same thing happens from the mobile form in the user's profile.

I looked through the code in sms_valid.module and saw that line 12 reads:

* Implement hook_sms_validate()

So, I created my own _sms_validate() function but this too, is not fired.

Having looked deeper, I can't work out where these functions are actually being called from because the functions called upon user submit from the profile are:

sms_user_validate_number
sms_validate_number
specific functions in whatever gateway is configured
...then the form is returned to the user.

So, how do I get the validation module to fire, or the international validation module, or my custom validation function to fire?

Many thanks,
Crom