They're working the phones as we speak. The problem is this time of year there's a few teams who are still stuck in the middle between buying and selling. Also, if teams know you're desperate, they're going to demand more in return.
Take the White Sox for example. They have talent on that roster but are underachieving big time. They're going to wait until closer to the deadline to start selling in case they get hot. It's a really weak division after all.
The Phillies are in the same boat. Same with the Guardians, Red Sox, Giants. A couple others as well.
All of these teams have guys (both arms and bats) who I've heard Houston has checked in on. Alex Verdugo, Joc Pederson, Michael Conforto, Zach Wheeler, Aaron Civale, Eloy Jimenez, Lucas Giolito, etc. Things will become a lot more clear over the next few weeks and we should be able to narrow those names down.
The Cubs are almost guaranteed to be sellers and I know for a fact Houston is extremely interested in Cody Bellinger. I've heard Drew Smyly's name tossed around as well. The Cubbies will wait until closer to the deadline to sell.. which is smart, because they'll likely get more in return with more teams in a bidding war.
Also please keep in mind "checked in on" doesn't necessarily mean a strong level of interest. The front office is just doing their due diligence and seeing what's out there.