Well, found something and thought of sharing. Looks like most of the headaches and rear-pains can be prevented by reading, rather well-documented data-sheets, app-notes.
I am guessing that most people felt pain when they assumed SIM900X series to be a "drop-in" replacement for SIM300X series, which it is not, even if the brochure / website seems to allude to that possibility. Read it as "replacement", not exactly "drop-in replacement". There are enough differences in the B2B connector, electrical differences in voltage thresholds/max's, and AT command set, startup string, to warrant a thorough read of the specs.
Anyway, I am trying to order some SIM900's as they don't seem to be as easily available in retail, as the SIM300's and will test this further and report back.
In the meantime, if anyone knows where SIM900A and SIM900S can be purchased in Bangalore, India - would appreciate if you please let me know. Currently tried all usual routes, but stock of those 2 models / parts, seem to be elusive.