Getting 403 Forbidden when trying to register Cisco SPA3102 ATA

I am trying to REGISTER the Cisco SPA3102 with the Asterisk/FreePBX. I have followed the configuration steps I found on this site and many other sites too. However, the ATA will not register with the Asterisk/FreePBX. I keep getting SIP 403 Forbidden.

My config is as follows:

sip.conf

[from-officepstn]
context=from-trunk
dtmfmode=rfc2833
secret=officepstn
type=user
username=officepstn

[officepstn]
dtmfmode=rfc2833
secret=officepstn
type=peer
username=officepstn
host=
port=5061

When I trace the Registrar message on the Asterisk server I get this:

<— SIP read from :5061 —>
REGISTER sip::5060 SIP/2.0
Via: SIP/2.0/UDP :5061;branch=z9hG4bK-658baa71;rport
From: <sip:[email protected]<Asterisk_ipAddress>:5060>;tag=3f9bf1c25031fafo1
To: <sip:[email protected]<Asterisk_ipAddress>:5060>
Call-ID: [email protected]
CSeq: 32708 REGISTER
Max-Forwards: 70
Contact: <sip:[email protected]:5061>;expires=3600
User-Agent: Linksys/SPA3102-5.1.10(GW)
Content-Length: 0
Allow: ACK, BYE, CANCEL, INFO, INVITE, NOTIFY, OPTIONS, REFER
Supported: x-sipura, replaces

<------------->
— (12 headers 0 lines) —
Using latest REGISTER request as basis request
Sending to : 5061 (NAT)
rixbox1*CLI>
<— Transmitting (NAT) to :5061 —>
SIP/2.0 403 Forbidden
Via: SIP/2.0/UDP :5061;branch=z9hG4bK-658baa71;received=;rport=5061
From: <sip:[email protected]<Asterisk_ipAddress>:5060>;tag=3f9bf1c25031fafo1
To: <sip:[email protected]<Asterisk_ipAddress>:5060>;tag=as4dd64572
Call-ID: [email protected]
CSeq: 32708 REGISTER
User-Agent: Asterisk PBX
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, SUBSCRIBE, NOTIFY
Supported: replaces
Content-Length: 0

Could anyone help me get the ATA to register with the Asterisk/FreePBX please?

Thanks