[Asterisk-Users] What does 'Forbidden (From header is not a Trust host or gateway)' mean?

[Evert Meulie]

 From a 'sip debug':

Sip read:
SIP/2.0 100 Trying
From: "Evert"<sip:[username]@[my ext. IP]>;tag=as6e18534e
To: <sip:[dialled number]@[SIP server of VoIP provider]>
Call-ID: 6cbf41c25281f08b2e7bbc5043061975@[my ext. IP]
CSeq: 102 INVITE
Via:SIP/2.0/UDP [my ext. IP]:5060;branch=z9hG4bK4fd1045b
Content-Length:0
 
 
7 headers, 0 lines

Sip read:
SIP/2.0 403 Forbidden (From header is not a Trust host or gateway)
From: "Evert"<sip:[username]@[my ext. IP]>;tag=as6e18534e
To: <sip:[dialled number]@[SIP server of VoIP 
provider]>;tag=87f2a0d5-13c4-4146e24d-1a7b91cc-796f
Call-ID: 6cbf41c25281f08b2e7bbc5043061975@[my ext. IP]
CSeq: 102 INVITE
Via:SIP/2.0/UDP [my ext. IP]:5060;branch=z9hG4bK4fd1045b
Content-Length:0
 
 
7 headers, 0 lines
    -- Got SIP response 403 "Forbidden (From header is not a Trust host 
or gateway)" back from [SIP server of VoIP provider]
Transmitting:
ACK sip:[dialled number]@[SIP server of VoIP provider] SIP/2.0
Via: SIP/2.0/UDP [my ext. IP]:5060;branch=z9hG4bK4fd1045b
From: "Evert" <sip:[username]@[my ext. IP]>;tag=as6e18534e
To: <sip:[dialled number]@[SIP server of VoIP 
provider]>;tag=87f2a0d5-13c4-4146e24d-1a7b91cc-796f
Contact: <sip:[username]@[my ext. IP]>
Call-ID: 6cbf41c25281f08b2e7bbc5043061975@[my ext. IP]
CSeq: 102 ACK
User-Agent: Asterisk PBX
Content-Length: 0