However I would like make the following suggestions:
2) In emails and letters, I would recommend the use of the word “regarding” strictly for the opening sentence, when reminding the recipient why you are writing
eg.
Dear Matthew,
I am writing regarding your modem’s warranty, as I have confirmed that lightning damage can not be covered under the terms…

4) It is even better to say that “the situation *is regretable*” which is kind of the verbal equivalent of “we are sorry for any inconvenience caused”. Call the situation messed up or screwed acknowledges the issue but gives the impression

6) Avoid asking questions such as these as much as you can! Instead use ask more relevant questions which engage the caller: “Has that helped??”, “Do you think you can to that the next time?”, “Are you feeling more comfortable with the software now Mrs Bates?”

DON’T say “please come again” – it is polite but rarely used and with an Indian accent you will make people think of Apu from the TV show the Simpsons.

14) I suspect you have mis-heard: “a quarter past one [o'clock]” or “a quarter to one [o'clock]” which mean 1:15 and 12:45 respectively. These are commonly used.

15) When “THE time” is used, one is usually talking about the current point in time. That is, “Do you have the time?” (ie. Can you tell me what the time is?) and “Do you have time?” (ie. Can you spare some time to …?) meaning very different things!

23/24) I don’t understand what you mean in this point, a phrase like: “Your package was shipped yesterday” is the convention, as is “we will send the invoice along with the package tomorrow”. Other phrases are used when not speaking relative to the current day – eg. “We do next business day shipping for all orders”, “I am sorry for the mix up, the marketting campaign started only one day prior to your order being placed and my colleague must not have been aware the campaign had commenced.”

Suggestion for #25 (not specific to Indian call centres): Do not quote turn-around times in hours. One “business day” is not the same as “24 hours” (nb: also do not attempt to say “business hours” for this is open to more interpretation and hence is less clear)

The only tips I might comment further on are 2 and 8. Most Americans will understand use of “regarding” or “pardon?” Problems understanding “pardon?” may have more to do with accent. The second syllable has to end on a very precise tone in relation to the first syllable. So you may be correct to say it’s safer to use “could you please repeat that?”