Customer edit - Local call charge should not be updated in minus value.

Steps to reproduce :

1. Login as admin
2. Go to account menu
3. Click on customer
4. Open edit page
5. Set minus value in Local call charge field and save
6. Then verify customer is saved successfully 

Actual result :

 > SIP2SIP call are working fine but also minus value is displayed in CDRs after call.

Expected result :

  >  Local call charge should not be updated in minus value.