We have two simple utilities that allow one to experiment with "dumb" bit-
shuffling conversion between AMR 12k2 and EFR codec formats, to explore
capabilities and limitations of this approach.

gsm-amr2efr reads an AMR speech recording in RFC 4867 storage format (the common
.amr format) and converts it to EFR in gsmx format.  The AMR input to this
utility must consists of MR122 frames only - no other AMR modes, no SID and no
NO_DATA gaps.  The intent is that one can take a starting speech sample in WAV
format, encode it into AMR with amrnb-enc from opencore-amrnb (by default that
utility produces MR122 encoding without DTX), and then convert the AMR output to
EFR with gsm-amr2efr.  One can then encode the same starting-point WAV speech
sample with gsmefr-encode (matching official EFR from ETSI) and compare the two
EFR outputs.  When you do this experiment, you will see that the two EFR outputs
will be different (you can then analyze encoded speech parameter diffs with
gsmrec-dump), but each version can be fed to an EFR decoder, resulting in
OK-sounding speech.

gsm-efr2amr performs the opposite conversion: it reads an EFR session recording
in gsmx format and converts it to AMR storage format.  The input to gsm-efr2amr
is allowed to contain Themyscira BFI markers in addition to EFR frames; these
BFI markers will be turned into AMR NO_DATA frames.  The same input can also
contain EFR SID frames - however, gsm-efr2amr will not detect them and won't
give them any special handling, instead they will be bit-reshuffled into MR122
just like EFR speech frames.  The result of such "dumb" conversion is invalid
AMR, and when you decode it with amrnb-dec, you will hear some strange noises.