It is a limitation of the CDMA network used for voice calls. The GSM network used by T-Mobile doesn't have the same limitation. You would have to check the conference call faqs, but you might have more options if the numbers you want to call are all on Verizon Wireless phones and have the Advanced Calling feature enabled on them.
I'm most definitely NOT a VZW employee. If a post answered your question, please mark it as the answer.
