When an Account Phone is updated, put the old phone number and new phone number into the Account Description field.

trigger AccountTrigger on Account (before update) {
    for (Account a : Trigger.new) {
        Account old = Trigger.oldMap.get(a.Id);
        if (a.Phone != old.Phone) {
            a.Description = 'Old Phone: ' + old.Phone + ', New Phone: ' + a.Phone;
        }
    }
}