--- main/format_cap.c	(Asterisk 13.5.0)
+++ main/format_cap.c	(working copy)
@@ -469,6 +469,7 @@
 
 	if (result && result->framing) {
 		framing = result->framing;
+		ast_format_set_current_ms(result->format, framing);
 	}
 
 	return framing;
--- main/format.c	(Asterisk 13.5.0)
+++ main/format.c	(working copy)
@@ -51,6 +51,8 @@
 	void *attribute_data;
 	/*! \brief Pointer to the optional format interface */
 	const struct ast_format_interface *interface;
+	/*! \brief Current length of media that can be carried (in milliseconds) in a frame; for example the negotiated ptime in SIP/SDP */
+	unsigned int current_ms;
 };
 
 /*! \brief Structure used when registering a format interface */
@@ -371,6 +373,21 @@
 	return format->codec->maximum_ms;
 }
 
+unsigned int ast_format_frame_blocks(const struct ast_format *format)
+{
+	if (format->current_ms) {
+		/* ceil (without double), required by Speex (RFC 5574 section 5.4) */
+		return (format->current_ms + ast_format_get_minimum_ms(format) - 1) / ast_format_get_minimum_ms(format);
+	}
+	return (ast_format_get_default_ms(format) / ast_format_get_minimum_ms(format));
+}
+
+void ast_format_set_current_ms(struct ast_format *format, const unsigned int ms)
+{
+	//ast_log(LOG_NOTICE, "%u, %s\n", ms, ast_format_get_name(format));
+	format->current_ms = ms;
+}
+
 unsigned int ast_format_get_minimum_bytes(const struct ast_format *format)
 {
 	return format->codec->minimum_bytes;
--- include/asterisk/format.h	(Asterisk 13.5.0)
+++ include/asterisk/format.h	(working copy)
@@ -358,6 +358,8 @@
  * \return minimum expected bytes in a frame for this format
  */
 unsigned int ast_format_get_minimum_bytes(const struct ast_format *format);
+unsigned int ast_format_frame_blocks(const struct ast_format *format);
+void ast_format_set_current_ms(struct ast_format *format, const unsigned int ms);
 
 /*!
  * \brief Get the sample rate of a media format