Surely your line quality will improve if the job is done better. The cables should not be exposed to air & water, but unfortunately BSNL guys do not care, they will do most pathetic cable job and even join cables by stripping and twisting the cable and leave the strands exposed. Ideally the joints should be made with the splice joints like these:
Aliexpress.com : Buy 100PCS/BAG UY2 Connector for telephone cable/copper cable splice joint connector from Reliable connector male suppliers on Shenzhen Qin Technology Co., Ltd. | Alibaba Group
these are available in the local market where telecom cables & connectors are sold.
When you say teflon cable, we are only talking about the jacket or a strength member made of teflon to give the cable strength. It would not help if you make the same shabby joints as with earlier cable. I think just using proper splice connectors will help you.