Hi Bjørn,
It does look like an issue in the UI when MANUALLY inserting the name instead of using the lookup or clicking the quick select drop down.
When doing it manually on our QA for your code, it does not find the phone number, etc. Looking at the query used, it is searching for the caller name instead of the person_id in the query which is not correct.
If you click the name in the drop down or search in the lookup, it finds the phone number. Please try that on your system to confirm. I also note the same when I use one of the names you indicated worked. I suspect different methods are being used to insert the caller name.
Do you get the same error if you use the lookup to find that user?
Please log ticket for this as it also looks to not find the phone number in latest 15.4 when manually inserting.