In deciding VOIP billing software you need to make a decision that you want to use the Sw for Telecom operation or you want to modify , support upgrade the billing software along with your telecom operations.
Pl. keep in mind that software support modifications is different from telecom operations. No company can do both.
If you want to be in only core telecom operation you need to go for quality and reliable biling softwate and take the support from software venders.