If you know the answer, please do leave me a comment about how to set it up. But, believe me when I say- Airtel have no clue about it themselves.
  
  Originally, not an Airtel customer and frustrated with Idea’s hold-on-call that lasted for a good 10-15 minutes everytime I called them, I thought of doing my first MNP(Mobile Number Portability) to Airtel. 
  
  I had a good history with Airtel and was very impressed by their QoS- a term that doesn’t exist in their dictionary now.
  
  My request was as simple as activating voicemail. Who wants to get a call from office at 4 am when he/she has reached home at 2 am? Yes, unfortunately, that was my poor story way back in March. But again, what if something got messed up terribly? I needed a workaround and thus subscribed to Rs. 20/- a month SMP(Smart Mail Plus). Thus began my frustration.
  
  Trust me when I say that I have followed the correct escalation matrix :-
  a) Call to Airtel Customer Care (121) at 50p for 3 minutes. Roughly about 2 hour
 b) Talk with floor supervisors. Roughly about ½ hr@50p per 3 minutes.
 c) E-Mail to customer care: 121@in.airtel.com. Roughly about 6 times.
 d) Escalation e-mail to South Nodal team. Once
 e) Visiting the nearest Airtel Store. They know just one thing- Dial 121 from the store.
 
 Finally
  f) Threatening to disconnect call and go for an alternative service provider. 3 times.
    
 Its utterly unbelievable folks but they’d rather lose a customer than take a sincere effort to resolve the issue. They have stopped replying to my e-mails or calling me now to take the issue to resolution.
   
 In a nutshell, it’s been a frustrating experience. I am going for a MNP again tomorrow, not for a VMS (now that I have changed my job & it’s not required) but only as a way of peaceful protest against a now sub-standard bunch of so called technical & customer service executives of Bharti Airtel.
  Sunil Mittal is a rich guy. He would not mind losing a customer who paid Rs. 1422.33 as a “parting gift” to the company. That is just Rs. 17067.96 a year…
  But, remember Airtel- HFZ hai !
  
  
 UPDATE
  
 For you reference, Smart Mail Plus procedure is as follows:
 Now MNP Port in customers can use smart mail service on 52555. Customer to dial USSD Short Code *321*671# for activation.
 Activate:
Call 52668 (toll free) to activate or deactivate
 Charges:
Monthly rental of Rs 15/- for existing customers. Rs.20/- for new activationRetrieving 75ps, Call 52667 (for settings) @ Rs 3/min
 Features:
- Call gets diverted to Voicemail(VM) under the following conditions
When the customer is out of reach
When Mobile is Switched Off
If unreachable
- Smart mail working condition when in roaming
In not reachable mode 
Switched Off mode
 - Get Missed Call Information(MCI) thru sms under the above conditions:
- All VM notifications or Missed call Information notifications will include mobile number and time.
- Retrieval of all VM on IVR 52666 or retrieval of a particular number VM on IVR 52666** (specified in sms).
- Retrieval of messages while in Roaming or even from a local Landline
 Retrieval:
- Call 52666(75ps) for retrieval of all VMs or 52666** as specified in the sms for a particular voicemail
- For Landline retrieval call 9945501491, enter mobile number and password* to access VMs
- Customer can retrieve while roaming by calling ten digit number 9945501491(roaming rates) after password* verification. 
 For Settings:
- Call 52667(Rs3/min) 
- Set your Password
- Record your personalized message or select a pre-recorded message
- Set messages as default message, Person specific message or Alternate message(define time period)