Why my outgoing VoIP calls are sometimes that bad?

I have a Cisco SPA122 ATA Router in Bridge mode where I connect my analog phone. Turns out that usually on incoming calls, I don't have any issues: connection goes smoothly, the caller hears me perfectly and so do I.

The issue is that sometimes (mostly, though), when I'm the caller, the callee hears me with so many interruptions that they cannot understand anything and we have to hang up. Other times it works just well.

I've tried really anything, so if someone has a tip on what could be the issue, I'd really appreciate any fresh idea. Here are some facts of what have I tried and results:

• With a softphone, everything works smoothly even on outgoing calls, always (using GSM codec).
• I've tried both with a Linksys ATA PAP2 device and a Cisco SPA122 ATA Router, same happens with both.
• I've tried changing the RJ-45 cable that connects the router and the ATA device, no result.
• I've tried switching to another VoIP provider, same happens.
• Usually both the analog phone, the VoIP ATA device, the router and a SmartTV are very close together. I've tried putting the ATA device far from them, same result.
• Of course I've tried switching between different codecs: G711u, G711a, G729... same result.
• Ports 5060/UDP and 10000-30000/UDP are open in my router device.
• I've even tried activating QoS to priorize voice calls, no effect.

The only thing left I haven't tried is replacing the router device, but before doing so I prefer to place that question to know any additional possible issues.

I've also considered this could have something to do with WiFi interferences, or even some other devices interfering together, but I'm not quite sure if this even makes sense. Any other ideas?

Thanks.

---- EDIT 11/04/2014 ----

I've made a "drastic" experiment: I've disabled WiFi in my router's config and surprise, outgoing calls work perfectly! Even with P2P programs working hard and other downloads.

I've been trying this config for 2 days and it hasn't failed at anytime. So what's the logic of this? Might WiFi interfere with the ATA device even when this latter doesn't use WiFi but a RJ-45 connection to the router? Or might it be that the WiFi signal is interfering with my neighbor's and making the signal that weak in my router that it makes fail the VoIP call?

---- EDIT 12/04/2014 ----

I've read the whole @harrymc's suggested article, and though my cable-modem is not the same brand, I've tuned up several parameters (for instance, deactivating b+gWiFi mode and leaving just n). Also, the QoS seems pretty much complex in the mine (Tp-Link TD-W8951-ND), I already had a configuration to priorize VoIP but I've also included one to priorize RTP.

Unafortunately, I can't find the WMM option in this router (at least not with this name or any related). But thanks to that link, I've also discovered an additional parameter, ATM QoS, which was set to UBR which seems to not be the recommended for protocols like VoIP. So I changes it to be CBR, will try this way to see how it works.