No audio on incoming call, outgoing calls work fine

I have a remote extension that I have finally gotten working - for the most part. You can place calls from the phone with no problem. You can dial the extension and the remote phone rings, but when you answer, there is no audio and the calling party cannot hear you. This is the error in asterisk:

[May 19 10:51:05] WARNING[27215]: chan_sip.c:3673 sip_write: Asked to transmit frame type 64, while native formats is 0x4 (ulaw)(4) read/write = 0x4 (ulaw)(4)/0x4 (ulaw)(4)

How can I resolve this error and enable communication for incoming calls?

That warning (not error) is quite likely unrelated. You need to provide the contents of the SDP exchanged during the SIP call setup, although you should also look at the firewall settings.

I was able to resolve the issue for the time-being. There were 3 codecs enabled, I disabled 2 of the 3 and was able to make the call. I am guessing that there is something else in the config that selects the appropriate format based on the call that was not applying to this extension. I have reenabled all codecs and am now able to make calls after moving the extension to a different context.