allocation of input service tax

I am working for a tata dealership. It is dealing in vehicles and servicing of vehicles. On servicing of vehicles service tax is payable and input can be used. However, there are common expenses such as "telephone expenses" on which input though available but can not be bifucated as to how much used for provided for taxable services and how much for sale of vehicles. Please suggest how to allocate input services tax on common expenses.

Thanks & Regards

Puran Didwania