GST Invoice

As the billing period is of March-18, you take account the bill in the month of March-18 by making provision of telephone expenses.

As per Section 16 of the CGST Act,   

(1) Every registered person shall, subject to such conditions and restrictions as may be prescribed and in the manner specified in section 49, be entitled to take credit of input tax charged on any supply of goods or services or both to him which are used or intended to be used in the course or furtherance of his business and the said amount shall be credited to the electronic credit ledger of such person.

(2) Notwithstanding anything contained in this section, no registered person shall be entitled to the credit of any input tax in respect of any supply of goods or services or both to him unless,-

he is in possession of a tax invoice or debit note issued by a supplier registered under this Act, or such other tax paying documents as may be prescribed; he has received the goods or services or both.

So if you take ITC of the said bill in the month of March, there will be violation of the said provisions.