This patch will probably make it work but the bug is actually in the phone, the…

This patch will probably make it work but the bug is actually in the phone, the patch is simply tolerating the bad behaviour.  You are correct about the a=sendonly missing, this was fixed in a later revision of the polycom firmware.  I suggest that even if this patch works, that you update your phones to a newer firmware, preferably the most recent.

src/mod/endpoints/mod_sofia/sofia_glue.c

@@ -4345,7 +4345,9 @@ uint8_t sofia_glue_negotiate_sdp(switch_core_session_t *session, const char *r_s
 		}
 	}
 
-	if ((m = sdp->sdp_media) && (m->m_mode == sdp_sendonly || m->m_mode == sdp_inactive)) {
+	if ((m = sdp->sdp_media) && 
+		(m->m_mode == sdp_sendonly || m->m_mode == sdp_inactive || 
+		 (m->m_connections && m->m_connections->c_address && !strcmp(m->m_connections->c_address, "0.0.0.0")))) {
 		sendonly = 2;			/* global sendonly always wins */
 	}