That's right. Here's why...
ISDN transmission is possible through the use of data compression. What is data compression? Well, it's what makes an MP3 file so small compared to a full resolution file. Some very smart computer programmers and acousticians came up with a way to reduce file size by removing redundant information, or information that is "masked" by other sounds. This can work well, but it has its limitations.
Like I mentioned, with data compression much of the data is thrown out; gone forever. That's fine if the algorithm did a good job, and if the audio doesn't need to be manipulated later, but what if I want to EQ that voiceover? Some frequencies that I may wish to enhance, have been removed by the data compression, and so the manipulated audio doesn't reach it's full potential.
The broadcast chain is another problem with data compressed audio. There are several steps in the broadcast chain after the mix occurs. Several of these could possibly employ data compression of their own, so now you are removing data from an already skimped down version of the audio.
"But we've been doing ISDNs for years!"
Yes, we have. And it does work, and you probably don't notice that anything is wrong. But we don't have to do it this way. Internet connections are so fast now that it really makes more sense to simply record where the talent is, and send the files via ftp. It's cheaper than booking two studios with ISDN capabilities, and it will sound better.