Israel has an intelligence agency that's generally recognized to be quite competent. I'm sure it would have taken them approximately 5 minutes to learn from their many spies that a new form of communications is being used.
The IRGC itself has clearly and spectacularly infiltrated; read the details of the Haniyeh attack. So, yeah, at this point I don't think anyone in the IRGC network can trust anybody else. The messaging here is pretty intense.
Theoretically, pagers are simpler devices, meaning it would be much easier in principle to analyse both hardware and software to check for issues, unlike a mainstream phone OS which has a bigger software attack area and can have at least some zero day attacks known to a state level actor like Israel.
Although, if it really was explosive inside the pagers it seems Hezbollah didn't do this.
I mean Israel is behind Pegasus, so it's not exactly a secret that they can get into any cellphone. Israel didn't need to infiltrate Hezbollah, they just needed publicity.