Implement bandwidth-efficient AMR payload as per RFC 4867 Section 4.3
Unfortunately the 3GPP has missed to specify which of the two variants are to be used on the AoIP interface. While 3GPP TS 48.103 specifies almost every other detail, it doesn't contain any statement on which of those variants shall be supported by either BSC nor MSC/MGW on AoIP.
The osmocom stack so far implements exclusively octet-aligned mode. However, some MSC/MGW seem to be implementing [only?] bandwidth-efficient mode.
It would be possible to add OsmoMGW support for bandwidth-efficient mode and then have it convert between the two modes such as octet-aligned on Abis and bandwidth-efficient on A. Identification should happen via the MIME type within the SDP (within MGCP).
OsmoBSC could then have a setting which determines the MIME type to be specified for the A-side RTP connections.