To accept Form 15H, the Bank will require PAN of your Grand Mother. The very fact that they have deducted TDS at 20% indicates that your Grand Mother either had NO PAN or had not provided it to the Bank.

One has to file ITR to claim the TDS, if they are not taxable. As she has expired, a legal representative will be required. 

Please consult a CA.

When was the actual Tax deducted ?  (Exact date)

On the date of deduction of tax, in whose name /s the deposit stands.

Your father can claim TDS providing the particulars and informing that the amount was given for her use independently and the TDS deducted may be treated in his account, as he is legal heir and also joint account holder.  TDS is always advance tax.

If the deposit is substantial, and if your father is already a higher tax payee, always better to consult a Chartered Accountant.

Also file RTI Application to concerned ITO and seek specific clarification as follows::

Application dt.1st Aug,2016 seeking information under RTI Act filed before

CPIO:  Income Tax Officer, Ward No...............(full address)

RTI Fee: Indian Postal order No...for Rs.10/- drawn fvg.Accounts Officer, ITO payable at.....

Name of applicant::............................

Brief facts: A Fixed deposit with joint account was held in the name of two persons, and in absence of form no.15H, 20% tax was deducted from the interest by the Bank in the name of first depositor .  The first depositor has expired and at present the second depositor is sole beneficiary of the deposit.

Information solicted.

1.Please provide laid down procedure for getting refund of the TDS deducted by the bank, as the depositor is now deceased, and the other depositor is sole beneficiary of the account.

Applicant.