daniel,
it is possible to call both keep_codecs_by_name and keep_codecs_by_id,
since 101 does not have name:
v=0.
o=twinkle 1548176998 229186862 IN IP4 192.98.102.10.
s=-.
c=IN IP4 192.98.102.10.
t=0 0.
m=audio 8000 RTP/AVP 99 97 8 0 3 101.
a=rtpmap:99 speex/32000.
a=rtpmap:97 speex/8000.
a=rtpmap:8 PCMA/8000.
a=rtpmap:0 PCMU/8000.
a=rtpmap:3 GSM/8000.
a=rtpmap:101 telephone-event/8000.
a=fmtp:101 0-15.
a=ptime:20.
a=zrtp.
-- juha