As opposed to my sib comment, I could see (theoretically, not saying this is what the original logic was) some justification to deal with intermittent line breaks or connection issues - if one side can keep the call open, then a wind gust breaking the connection for a couple milliseconds somewhere between the two parties won't cause the whole call to end. From a customer point of view, it's more resilient and ends up with fewer dropped calls.

I could also theorize about the different switching actions going on, where up until the other party picks up there's already only one phone on the line, but that's getting into phone system/phreaking stuff that is way out of my depth.