I called airtel customer care and asked them. According to the information given me, once your bundle expires, there is no way to re-activate it, not even with a top-up. The info I got was that 10mb, 25mb (and the 50mb they dont know they have) have a 'lifespan' of one month while the other bundles have a 'shelf-life' of two months.
I hope they sort that out soon (re-activation of expired bundles) otherwise that's what we have to work with.
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This is your last chance. After this, there is no
turning back. You take the blue pill - the story ends, you wake up in
your bed and believe whatever you want to believe. You take the red pill
- you stay in Wonderland and I show you how deep the rabbit-hole goes.
~The Matrix
Eva Kimathi