I'm sorry, User Registration Solution Pack uses the Client and Server Side validation in CSS Form builder for the telephone format. You would need the CSS Form builder Extension to be able to modify the formats.
Unfortunately, CSS Form builder is not compatible with DW8. Only CS3 and above.
If you wish, you could remove the phone number validation code from the registration page.
The Client side validation is applied to the onBlur event of the form element:
change:
<input name="UserPhone" type="text" class="WAATKTextField" id="UserPhone" onBlur="if (document.getElementById('PhoneServerValidation')) document.getElementById('PhoneServerValidation').style.display='none';WAValidatePN(document.forms[0].UserPhone,'- Invalid phone number',false,true,'x (xxx) xxx-xxxx',document.forms[0].UserPhone,0,true)" value="<?php echo(ValidatedField("usersRegistration","UserPhone")) ?>" size="32" />
to:
<input name="UserPhone" type="text" class="WAATKTextField" id="UserPhone" onBlur="if (document.getElementById('PhoneServerValidation')) document.getElementById('PhoneServerValidation').style.display='none'" value="<?php echo(ValidatedField("usersRegistration","UserPhone")) ?>" size="32" />