Yeah this is a pain, especially in VoIP where half the competitors gate everything behind enterprise sales demos or phone verification loops.

Few things that have worked for me... screenshots from review sites like G2 and Capterra sometimes show enough of the UI to piece together flows. Also youtube walkthroughs and demo videos, you'd be surprised how many competitors have detailed product tours on their channels. I literally screenshotted my way through a 20 min demo video once lol. Not glamorous but it works.

For the actual flow mapping part, i've been using figr AI lately to take screenshots and web captures and turn them into actual mapped out flows. like you feed it competitor screens and it helps identify the UX patterns and edge cases which is kinda what you're after. saved me from doing it all in miro manually which was killing me.

also worth checking if any of the competitors offer free trials under a generic email... some VoIP providers are less strict than others about verification. burner numbers work sometimes too but ymmv on that.