To release numbers from your account please to the Admin > General Settings > Outbound Numbers > choose a number and use the 3 little dots on the right > delete.
Please keep in mind, this is just possible if the numbers are older than 6 months (min. holding time).
If numbers are NOT older than 6 months please send over the phone numbers to firstname.lastname@example.org and include your account manager in CC.
Please take into consideration that your phone number might have an inbound route or queue on it. This will be removed automatically once the number is released.