Hi Anne.
Personally it sounds to me like someone has intentionally been interfering with the Manual Configuration of the number ranges via Transaction BUCF.
Although the next number maybe showing 9000050370, typically there is a small range of numbers already buffered in memory (Usually 10), hence you're hitting issues in the 900005036* range.
What I would do is locate the highest contact number on table BUT000. I would then add 20 to this and then adjust the Manual Configuration via Transaction BUCF to this new value.
I'm hoping this will resolve the issue.
Regards
Arden