Hi everyone

This is my first time configuring a router for use with VoIP and I'm at a crossroads where I'm not sure if I'm configuring things correctly or if there is a service issue. 

I have the D-Link setup with a sim card on Spark's 4G network for Internet access (the router is going into a location where a physical connection is unavailable). I have wireless and Internet access configured OK. The router has a RJ-11 port with a phone handset connected. I am running the latest posted firmware.

I can make calls and intermittently I can receive them but more often than not I get an IVR message saying that the person at extension x is unavailable. I have logged the issue with 2Talk - but so far haven't heard anything back.

My configuration of the router is default except for the following settings:

VoIP Line:
Name: voip1
Phone number: <my 2Talk number>
Username: <my 2Talk number>
Password: <my 2Talk password>
Codec Priority:
G792a 1st 20ms
G.711a-law 2nd 20ms
g.711u0law 3rd 20ms


VoIP Profile:
Name: 2Talk
Interface: usb0
Server IP / Domain Name: sip.2talk.co.nz
Port: 5060
Protocol: UDP
Enable Outbound Proxy: ticked
Outbound Proxy Address: sip.2talk.co.nz
Port: 5060
User agent domain:<blank>
User agent port: 5060
URL scheme: SIP
User parameter: none
Initial unregister: ticked (cannot untick)
Register expires: 3600
Session expires: 3600
SIP DSCP: EF - 46
Min-SE 180
Session Expires Refresher: UAC
DTMF Type: InBand (G.711)
DTMF Payload : 101
Enable RPORT: ticked (cannot untick)

RTP Features
RTP Port Lower: 49152
RTP Port Upper: 50000
RTP DSCP: EF - 46

Any help anyone can offer would be appreciated, I'm at my wits end, and not really sure of what all of the settings do. Is there a firewall rule I need to define to get this working?