Insight:
Voices are limited to one resource. When you utilize 5/5 voices and delete it, this means particular resource have exhausted that hard limit of 5 and you are not supposed to create again 5 voices therefore UI mentions 5/5 voices.
Solution:
Please try creating another resource under the same subscription. This should allow this time again you to create another 5 voices.
Best Regards,
Sunil Kumar