We look at a pre-date phone call as a way to chat on the phone, make sure there are two halves of a couple who actually want to talk to us, and to become a little acquainted with each other. If there is no lady available to talk on the phone when asked we will not meet a couple. It's a little different than one-on-one dating, because of the dynamics of couples, and the very real possibility of being stuck on a date with a guy whose wife "couldn't make it". The phone call is often a requirement for many couples in swinging. In our opinion it's not too much to ask. If someone forms an illusion of you and then is disappointed in person, then they haven't been swinging or even dating for long .
If you don't want to talk about sex on the phone or IM, just say politely but directly that you would rather leave those discussions for after you all decide you want to play after meeting in person. The exceptions to this are questions and answers about rules and preferences that can be deal-breakers, like whether you (or they) use condoms, whether both halves of the couples play, desires for girl-on-girl and similar things. We like to ask those questions before meeting someone, to eliminate wasted time on both sides.