Receiving phone calls on a Cox VOIP phone.

When I try to call a Cox VOIP phone from my landline with another phone company I get a recording "All Circuits are busy, try again later."  If I call from my cellphone it works fine.  I have called the tech support from my phone company and they cannot find any problems on my end.  The Cox VOIP customer called cox tech support and they said their equipment was working correctly also.  Obviously something is wrong.  Could you please double-check your equipment.