diff --git a/library/AMR_Types.ttcn b/library/AMR_Types.ttcn
index b4f044df8..5586a202d 100644
--- a/library/AMR_Types.ttcn
+++ b/library/AMR_Types.ttcn
@@ -33,7 +33,13 @@ type enumerated AMRFT {
 const integer c_AMRFT_len[9] := {12, 13, 15, 17, 19, 20, 26, 31, 5};
 
 function f_amrft_payload_len(INT4b amr_ft) return integer {
-        return c_AMRFT_len[amr_ft];
+	return c_AMRFT_len[amr_ft];
+}
+
+const integer c_AMRFT_bits_len[9] := {95, 103, 118, 134, 148, 159, 204, 244, 39};
+
+function f_amrft_payload_bits_len(INT4b amr_ft) return integer {
+	return c_AMRFT_bits_len[amr_ft];
 }
 
 type record RTP_AMR_Hdr {
diff --git a/library/OSMUX_Emulation.ttcn b/library/OSMUX_Emulation.ttcn
index c6999ded9..4538fdffa 100644
--- a/library/OSMUX_Emulation.ttcn
+++ b/library/OSMUX_Emulation.ttcn
@@ -374,12 +374,20 @@ private function f_txhandle_cid_add(OsmuxTxHandle hdl) runs on OSMUX_Emulation_C
 				log2str("No Space in TxHandleList for ", hdl.cid));
 }
 
+/* Generate correctly-sized AMR payload based on amr_ft, using tx_fixed_payload as a fill pattern. */
 function f_osmux_gen_expected_rx_rtp_payload(INT4b amr_ft, octetstring tx_fixed_payload) return octetstring {
+	var integer payload_len_bits;
 	var integer payload_len;
+	var bitstring payload_truncated_bits;
 	var octetstring payload_truncated;
-	var integer i;
+
+	/* Generate the AMR payload as a bitstring, since some formats don't end up in octet boundary: */
+	payload_len_bits := f_amrft_payload_bits_len(amr_ft);
+	payload_truncated_bits := substr(oct2bit(tx_fixed_payload), 0, payload_len_bits);
+
+	/* Now convert it as an octet string filling final padding with zeroes: */
 	payload_len := f_amrft_payload_len(amr_ft);
-	payload_truncated := substr(tx_fixed_payload, 0, payload_len);
+	payload_truncated := bit2oct(f_pad_bit(payload_truncated_bits, payload_len*8, '0'B));
 	return payload_truncated;
 }