[VoiceOps] Network Security Audit

I have been asked by management to find options to bring in an external auditing company to evaluate our network and provide appropriate recommendations. We are a small telecommunications company looking for firms with experience in Voice (VoIP) security audits, as well as general security experience appropriate for a Broadband ISP, with an internal corporate network. We're located in the southern US (Oklahoma). Public and private recommendations are welcomed. Thank You, -- Dan White