Welcome to Goodbudget! It sounds like you’re right on track to get your envelopes where you want them, but there are two things you’ll need to do so this pattern won’t pop up again next month.
First, you’re right in that the easy way to keep a zero Unallocated balance would have been to do an “envelope transfer” from one (or more!) envelope(s) into your fixed expenses envelope. That way the same amount of money is available, it just has a “new job”. You could also accomplish this by opening your Fill transaction and moving things around in there.
To fix this month’s envelopes you’ll need to subtract money from an envelope, either by adjusting the original month’s fill as mentioned above or by just transferring it to Unallocated through an envelope transfer.
For next month, you’ll need to be sure you’ve adjusted your budget to account for the cellphone and that any scheduled Fill takes that into account too.
Here’s an example if that wasn’t clear:
On July 1 I add $1000 (my monthly budget) to my empty Fixed Expenses envelope and $500 to Dining Out. On July 8 I realize I haven’t included my cellphone so I add $100 more to the Fixed envelope, but now my Unallocated balance is -$100; oops! If I had just transferred the $100 from Dining Out to Fixed, the unallocated would remain at $0 because I haven’t used any “new” money.
Now I change my monthly budget to $1100 for Fixed and $400 Dining Out, and be sure that any automatic Fills reflect the adjustments.
I hope this helps!