Fixing failed voip calls to mobiles from a Cisco SPA112 ATA

So, my old Billion adsl modem with a built in fxo port for voip went on the blink. I replaced it with a Cisco SPA112 ATA (in conjunction with a new ADSL modem which didn't have built in voip support).

However, after configuring up the SPA112 with my Internode Nodephone details I was getting some funny behaviour. Calls to landlines and Optus network mobiles would work ok, as would incoming calls. However, calls to Telstra mobiles were failing - the mobile phone would just come up with a call failed message and then the call would either go through to voicemail or the mobile would ring again (but would get call failure if answered).

I got the same behaviour when configured using the simple wizard and with the full Internode config for a Sipura ATA.

Eventually found a forum post from vange, who experienced something similar on Optus broadband with a Cisco voip phone.

It turns out the solution is to change the rtp packet size from the default of 0.030 to 0.020. The setting can be found under Voice -> SIP -> RTP Parameters -> RTP Packet Size.

After changing this it all seems to work and it's happy days again.