VoIP quality (outbound) poor

Question

Hi, 
 I installed a SG115W (running 9.409-9) at a client site a couple of months ago. 
 VoIP works (after creating a firewall rule) but I have never been able to get the quality to any decent standard. The problem appears on outbound voice only as when I call their office, they hear me fine but they will constantly 'clip' where I basically miss every fifth of a second of the call so they're mostly understandable but it's hard to listen too. Sometimes (like right now) it's very bad and not understandable. 
 
 I've disabled IPS and the three Anti-Dos/Flooding protections are not on. 
 I've disabled Advanced Threat Protection. 
 I've enabled QoS on the External (WAN) interface. They pay for 100mbit/40mbit but realistically it's more like 85/30. I currently have the QoS limits set to 100/40 but I guess I should change it to the realistic limits? 
 I've created several different traffic selectors (basically trying to capture all VoIP data). 
 I've given those traffic selectors guaranteed 5mbit/s of bandwidth. 
 All that and still no decent outbound voice quality. 
 It's definitely related to the Sophos as when they put their old ISP router in, the VoIP is great. 
 Any further suggestions?

TNT · Accepted Answer

I think I may have found my issue when I was taking these screenshots....specifically in Bandwidth Pools, I had never noticed the Bound to Interface option. I had originally created the bandwidth pools under the Internal interface. I have since re-created it under External (WAN) as pictured. 
 It's after hours right now so I won't be able to test this change with the client for another 12 hours. 
 
 I was able to test by calling their main number which goes to a night mode message...and it works!! Hoo-freaking-ray. When listening to the night message, I can enable the 'upper bandwidth limit' and set it to 1kb and the quality goes to absolute shit. I then disable that limit (and have a guaranteed bandwidth of 5024) and get crystal clear quality. 
 Of course, real test is still tomorrow when the network is seeing normal use but fingers crossed. 
 In the mean time, let me know what you think of the below. I was going a bit crazy so created multiple traffic selectors to make sure the traffic was being selected.

sachingurung · Answer

Hi TNT, 
 Configure Exception \ UDP Flood Protection for VoIP services. 
 
 Check if anything is dropped in the packetfilter.log and check in case of SIP, if the invite call is going through port 5060 or if it is a random high port. 
 Thanks

SaschaParis · Answer

a older thread, but check this