I only get through about once every five times I call that company.
The phone at that company is busy about 80% of the time.
この場合、次のような言い方ができますよ。
ーI only get through about once every five times I call that company.
「あの会社に電話すると５回に１回くらいしか繋がらない」
to get through「（電話が）通じる」
about once every five times「５回に１回くらい」
ーThe phone at that company is busy about 80% of the time.
「その会社の電話は80％は話し中だ」
５回に１回ということは80％とも言えます。
busy で「（電話が）話し中で」
ご参考まで！
It seems that that company receives so many calls that you can only get someone on the phone once every five tries or so.
こんにちは。
ご質問ありがとうございます。
"It seems that that company receives so many calls"＝「その会社はとても多くの電話を受けているらしく」
"that you can only get someone on the phone once every five tries or so."＝「5回に1回ぐらいしか誰かと話すことが出来ない。」
ご参考までに。
カナダ 
アメリカ合衆国 
