DC, I didn't find that quote in the book. Maybe you remember the part
Only thing I found is that Vito never used phones himself, he was afraid of his voice being recorded. But everyone else, Tom, Sonny, were talking business using falsely listed telephone.
DSC, Vito told Tom to check Tessio's and Clemenza's phones after the peace talk. Maybe he knew that they were less cautious than himself and could possibly talk business on the phone, at least make some meeting agreement etc.
Also, when Tessio makes contact with Barzini he phones Mike to appoint the date of their meeting. I think he would call.