"It's a relief to hear your voice on the telephone. - Okay, but "on the telephone" is a bit redundant. If you're talking to the person on the phone, then clearly it's "on the phone."
I am so worried about those air travels..." Not very natural. Try something like "I get so worried when people fly." or "I am so worried about air travel these days." or even "Flying always makes me nervous."
Student or Learner