If you are finding that some contacts are not able to hear you and others are in the same session, this may not be a Phoneburner issue, but an issue with the receiving end.
Some users will block calls they don't know with a service called RoboKiller that uses Answser bots. The bots sound like a real person and some of them will sound like someone saying they can't hear you.
If you consistently have trouble with users hearing you , please try clearing the cache in your browser and try dialing again. If the same numbers always have the same response, it may be an answer bot.
If you feel that it is an issue with Phoneburner, please contact firstname.lastname@example.org.
Or open a ticket from the support tab in your account. If you can turn on call recording and get a recording of calls where the user can't hear you, this is helpful.
Please provide the contact phone number, the date and time of the call and any other important information so support can troubleshoot.