Voicemail only pbx

I’ve been banging my head on this for a while now trying to figure out what is going wrong-
I have setup several extensions for voicemail the actual phones are physical phones on another switch that when called know to goto the voicemail pbx and when dialing *87 (555000MAIL) etc the switch sends the call to the pbx to check voicemail - now actually calling them and getting there to leave voicemail is not the problem - and I can see that when dialing *87 the call gets there to check the voicemail but all i can get is a fast busy tone/all circuits busy message - the resulting debug is telling me proxy authentication error but there should be no authentication? is there anything I can do to resolve this - any suggestions anyone could give me?
Examples:

inbound route
555000MAIL goes to misc dest *87 check users mailbox

inbound route
*87 goes to check mail–sent from switch as users phone number to pbx
the inbound route matches the users caller id and should send it to check voicemail
(if no caller id match then sent to leave voicemail)
these actually get sent to the pbx but result in:

SIP/2.0 407 Proxy Authentication Required
Via: SIP/2.0/UDP switch.ip.add:5060;branch=z9hG4bK-00102-bln453mr1idfx6t-0;received=Switch.ip.add
From: “CNAM RESTRICTED” sip:[email protected]:5060;tag=bln453mr1idfx6t-IPTrunk-922-17-23atswitch.ip.add
To: sip:[email protected]:5060;tag=as1d79f38b
Call-ID: [email protected]
CSeq: 102 INVITE
User-Agent: FPBX-2.8.1(1.4.22)
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, SUBSCRIBE, NOTIFY
Supported: replaces
Proxy-Authenticate: Digest algorithm=MD5, realm=“asterisk”, nonce="52632f02"
Content-Length: 0

I can call the actual number and after a certain amount of rings I get sent to voicemail where I can leave a message - no problem there