The word infrastructure doesn't drive the choice of article in this sentence, it's the word issue. Infrastructure functions as an adjective in this sentence, so we ignore it to choose or ommit the article.
we will be facing infrastructure issues is correct
we will be facing an infrastructure issue would also be correct
As you can see, it depends on the number of 'issues'.
Secondly, it would be most common in the US to say 'per our phone conversation'. Slightly less common (but not at all unusual) to say, 'per our telephone conversation'. I've never heard reference to 'telecom' or 'telephony' in the way you suggested. Neither one is grammatically 'incorrect', but either one would instantly mark you as not a native speaker...
Student or Learner