Magento version: 1.9 How to add the telephone field into registration form and make it required?
First, telephone no is Customer address type and it is not customer type attribute.
By default,if you want to enable telephone then make true getShowAddressFields() then you need to pull all address fields values street adress,country ,region,city etc.
an customer attribute to Customer entity type which will save the telephone to customer table.
Which will show on
customer registration,edit,checkout form.
See how to install an Customer attribute in magento Adding custom attribute to Customer
There it should no relation between billing/shipping telephone.
Also. need more detail checkout at http://excellencemagentoblog.com/blog/2011/10/02/customer-registration-fields-magento1-6/