If you want callers to be part of a podcast...ask them to email you if they want to be part of the show. Then you call them...
If you're not getting live, incoming calls, revise your strategy to focus on outgoing calls, from you to listeners. Use e-mail or social media to prepare ahead of time and get opinions going, then prerecord outgoing calls to people who've expressed interesting opinions and use them in the otherwise live production. Any time you finish a call, let listeners know that they can call right now, and see if the recorded calls prime the pump for more live calls.
This "problem" can be a blessing in disguise, especially if you're a one-man show. No need to screen calls, because you're making them and can edit afterward.
Be careful not to edit calls so tightly that they no longer feel natural.
I've done over 3,500 episodes of four different podcasts over the years, and more than 50% of the downloads have occurred
more than 30 days after they were first published. A show focused on current events without interviews of high-profile guests will have a higher percentage of listening close to release date, but check your stats and see when your downloads happen. You might be surprised, and it may change how you approach the show.
We tend to think of podcasts as "radio on the internet," but they're not; the usage models are completely different. Success will require exploiting different production techniques to optimize the medium.