increase dtmf volume (#208)

https://github.com/jambonz/jambonz-feature-server/issues/1272

lib/call-session.js

@@ -947,11 +947,13 @@ Duration=${payload.duration} `
           const code = arr[1];
           const arr2 = /Duration=\s*(\d+)/.exec(req.body);
           const duration = arr2 ? arr2[1] : 250;
+          const volume = 13;
           const dtmfOpts = {
             ...this.rtpEngineOpts.common,
             'from-tag': this.rtpEngineOpts.uac.tag,
             code,
-            duration
+            duration,
+            volume
           };
           const response = await this.playDTMF(dtmfOpts);
           if ('ok' !== response.result) {