Here are some references of BillingLabs users.
Covage is a telecom infrastructure operator focused on fiber networks. It deploys and operates FTTO and FTTH fiber networks on behalf of local authorities. Their instance of BillingLabs is the most shaken and proven to date.
Arkadin is a collaboration solutions provider that offers a complete range of Unified Communications solutions in the Cloud. SpikeeLabs maintained their billing solution among others.
Unyc is a french national-wide BtoB telecom operator. It provides a range of telephony, collaboration and cloud services, directly and indirectly through white labels. BillingLabs handles the invoicing of all these business model!