So I only ever found the second digit of the phone code by brute forcing after finding the other 3. Sadly, I don't want to spoil too much about it after FINALLY finding it courtesy of the hint system. But here goes.
The entrance to the room REALLY shouldn't require you to step through traps to get to it (Yes, even after the "helper mechanic"). Not to mention it's actually pretty hard to see on that particular tile set. It's also in a spot where you're pretty much NEVER going to just look.
Also, in the starting room, the Polypropeline IV will lose its glitter after first activation regardless of whether or not you used it. Maybe see if you can make sure that sparkle only removes AFTER using the entire bag. You know, in case someone went halfway before and wants the rest of it later.
It also occurs to me that another handicap option could be added, "starter implants". Basically, they give you +300, +500, or even possibly +1000 to your minimum size (up from 0). Less room for mistakes between rest rooms, and puts you on a pretty strict timer when combined with the Polypropeline IV, especially without the medipens. Although with that in mind, maybe that unlocks the code for the phone immediately?
I'm definitely excited to see more progress. Also at the prospect of "additional improvements" a certain trap promises, even though we can't reach the desired size yet.
P.S. - Already did a full run after draining the entire Polypropeline IV from the starter room.