NOTICE[11389]: rtp.c:1808 ast_rtp_read: Unknown RTP codec 100 received from
I've found that trying several times may work, but I also read changing your Sipura VOIP adapter settings as follows helps (but it still fails for me sometimes):
In adapter change the following under SIP/Advanced
Codec to G711a from G711u
Passthru Method from: NSE to ReINVITE
FAX CED Detect Enable: Yes
FAX CNG Detect........