The TLVs are documented in GobiAPI. I pass 0xff for the call ID, as the stock RIL appears to always do. I would guess it means "current foreground call." The call ID is returned in TLV 0x10, but I didn't implement parsing of that. Co-authored-by: Joey Hewitt <joey@joeyhewitt.com> Acked-by: Alexey Andreyev <aa13q@ya.ru>