diff --git a/lib/call-session.js b/lib/call-session.js
index c4f520f..60325fd 100644
--- a/lib/call-session.js
+++ b/lib/call-session.js
@@ -228,6 +228,7 @@ class CallSession extends Emitter {
                 ...this.rtpEngineOpts.uas.mediaOpts,
                 'from-tag': this.rtpEngineOpts.uas.tag,
                 'to-tag': this.rtpEngineOpts.uac.tag,
+                flags: ['single codec'],
                 sdp
               };
               const response = await this.answer(opts);