Student or Learner
1) Sometimes when you use the calling card to make international call, the call is low in quality.
2) The call was lack of clarity/low in quality, so make sure you can't hear anything before you hang up.
1) Sometimes when you use a calling card to make an international call, the connection is poor/low quality.
2) If/When the line/connection is bad, make sure you can't hear anything before you hang up.
The second isn't clear to me because you have the past and present tenses in, but the present seems more logical to me.