Heya, I appreciate the feedback you're giving me. As you say, this is a much larger scale project compared to Slimes, so it's easy for me to miss out on stuff like this.
So, on the second party member, you're almost there. In the sewer dungeon you're talking about, there's a path to the south that leads to the missing phone. The way is locked, but you can buy the lockpicking guide from the adventurer in front of the Waking Stones after the treasure-jacking scene and buy a lockpick off Sheya. Now, my main fear is that you might not be able to fight the boss, due to enemy scaling and the fact that you couldn't buy equipment to keep up. Honestly, you bringing this up may be the most valuable piece of criticism here, so I appreciate you bringing this up. I'm planning a much smaller update to fix a bunch of problems and now I'm going to add a more explicit hint to check there besides having players rely on to Tip Hotline (don't be scared of consulting it!).