Airtel — Extraneous billing on old Broadband Plan after availing a new Plan on same Phone line

Sir
I had an Airtel plan with the following details-.
Fixedline No. {{6 - - -}} Broadband ID : {{7 - - -}}_mh Rental ₹.399/- Speed 2 mpbs. Bills in respect of this connection used to be debited to my account through ECS. As I Wanted higher internet speed, I requested to update the plan. I was advised that the existing plan can not be updated and that I have to go for a new plan. I was suggested to have Airtel-UL-Basic 699 unlimitedGB(40 Mpbs/1024Kbps) COMBO Plan. I opted to avail the said plan paying yearly subscription ₹.8, 413/-. Accordingly, a new installation was made on 15.08.2020. Details of the new installation are as under-
Account No.[protected] CAF No.[protected] Airtel Fixedline No.{{9 - - -}}
Xtream Fiber ID :[protected]_mh Rental : ₹.699/- Plan Data : Unlimited GB
Plan Speed : 40 mpbs. Type of connections : New Installation DSL ID :{{8 - - -}}
Tariff Plan opted :Airtel-UL-Basic 699 unlimitedGB (40Mpbs/1024Kbps) COMBO Plan.
The installation was carried out on 15.08.2020. As such, my phone line on which I had my old Fixedline Number {{6 - - -}}, I got a new Airtel Fixedline Number {{8 - - -}}. However, bills in respect of the old Fixedline Number {{6 - - -}} continued to be debited to my account. At the same time I did not receive any bill in respect of the new Fixedline Number {{8 - - -}}(With nil amount payable). Bill in respect of the new connection started coming from 23.04.2021 only. I, vide my email dated 29.10.2020, pointed out that the New Plan despite having been installed on 15.08.2020, I continue to receive bills for the old plan. I quote my email for reference “I have got my old plan with above-mentioned number discontinued and have new plan installed on 15.08.2020. I, however, continue to receive bills as per the old plan. This is to bring the facts to your notice.” However, my said email seems to have been ignored.
My contention is that ever since installation of the new plan which was on the same phone line as the old one, bills in respect of the old one should have been discontinued. Even my email dated 29.10.2020 in this regard was overlooked. Under the circumstances, I have requested for refund of the bill amount of the old connection from 15.08.2020 to May, 2021 which works out to ₹.7135/-. I intend to attach Installation Report, Communication showing Bill Plan Details & Account Details, Bill in respect of New Plan, copy of my email dated 29.10.2020 and the last reply of Airtel in the matter for your reference, however the same are not acceptable in the system. I may send them by any other mode as per you advice.