Create a sequence (Your required Type) in xml
for example
<?xml version="1.0" encoding="utf-8"?>
<openerp>
<data noupdate="1">
<record id="seq_type_product_product" model="ir.sequence.type">
<field name="name">Product</field>
<field name="code">telephone</field>
</record>
<record id="seq_product_product" model="ir.sequence">
<field name="name">Product</field>
<field name="code">telephone</field>
<field name="prefix">NG</field>
<field name="padding">5</field>
<field name="company_id" eval="False"/>
</record>
</data>
</openerp>
after you write this code in corresponding python file.
_defaults = {'cust_id': lambda obj, cr, uid, context: obj.pool.get('ir.sequence').get(cr, uid, 'telephone'), }
t=0
this sequence started with NG00001...