When importing data in Dynamics CRM 4.0 there is no client side code triggered, so to make sure you get phonenumbers (or anything else for that matter) in a clean format, you might want to consider creating a plugin that runs server side and generates an import failure to let the customer know some data is not valid in his imports.
This way the user knows when something went wrong and the correct data will still be imported.
To make this plugin work there is the issue of the country prefixes and areacodes that exist. Therefore it is necessary to create two new entities: Country and AreaCode.
The Country entity needs to contains the telephone prefix of that country (eg: belgium = 32, Netherlands = 33 …)
The AreaCode is contains a link to the country entity and contains the phone areacode. (eg: 9 for Ghent, Belgium …)
Ones this data is in CRM we can use it in our plugin through the webservice.
In the end the phonenumbers will be formatted in an outlook compatible format.
eg: +32 (9) 265.74.20
When using the installer inside the project on CodePlex the plugin will be registered for the create and update messages for lead, account and contact.
You can find the plugin on CodePlex in our Dynamics CRM 4.0 Plugins project