Ask MTNL to provide wire from junction box to your flat, they will do if you OPT, while booking land-line, MTNL will charge Rs.250/- Officially but will manage & maintain free for life time. If you ask any pvt. guy or mtnl lineman in pvt. for rs.250/- he will do same but they will charge every time to manage & solve any issue related to that line (wire).
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Than ask secretary to find out wire for you from Junction, secretary seems to be Problematic culprit in every societal, especially in Mumbai?
I too have very long wire as our building don't have MTNL junction, problem is some time very good speed, some time, disconnect in between & reconnect. 1-2 time may be problem (dead) in monsoon but your complaint will attend promptly & than no problem even if heavy rain. My connection wire is nearly 10 meter in open air/water & sunlight, but it's by MTNL so I don't care as they will replaced/repair it promptly if any problem in future.
HTH