MTNL sucks, my signal strength went from excellent to weak, line attenuation skyrocketed, snr margin reduced too, now every time internet activity increases the connection disconnects and gets a new IP, the response time for browsing also has gone down-the-drain.
Raised a customer-issue case 5 months ago , re-requested it every month since then and no reply till now. I never expected a decently working line could go bad, seems like they have unqualified people working for MTNL.
I would say never use MTNL in metros where you will definitely have better options.
Changing and updating plans is not an option if one cannot even maintain a decent working connection.