I'm volunteer with no relationship to Cisco. I have even no support contract, thus no access to Cisco's support (we tried it, but we has been dissatisfied with the support provided by Cisco).
Just few years ago, at the times of Linksys department, the firmware developers (or at least employee with deep-in knowledge of matter) has been active here. Of course, their time has been so valuable to respond those RTFM questions, thus volunteers in this forum, including me, saved their time responding all those simple questions (or question responded already in the past). As the reward, engineers has been responding our questions (e.g. those responded by no one, or those that require in-deep knowledge of hardware design and firmware source code). If a bug has been discovered, they work actively with us on patch. I remember I created testing environment for remote engineer as well as tested (almost online) patched firmwares to verify the issue has been solved. God bless all of them. It has been reason to be here - I helped others to become satisfied customer, and Cisco, as a reward, helped me to solve my issues.
As Cisco dropped the Linksys division, it work no longer such way. Answers from someone wearing 
badge are so rare to be mentioned. I no longer asks here as no one will responds, also I don't place firmware bugs analysis here. It's waste of time. We are left alone here.
To say true, just few discussions provide an valuable experience to me (yes, this one is an example, so I burned a lot of hours on it) during past two years. It's rather matter of habit I'm still responding here.
Yes, I has been elected Cisco Support VIP twice. Yes, it's honor, but it's just honor. It doesn't mean I have access to anyone who can respond complex questions nor I have access to a resource you have no access to. OK, OK, I'm damned liar. As VIP I have free access to CTE (it's pretty expensive it you consider you are interested) - unfortunately, it doesn't cover SMB segment - thus it have zero value for the area I'm most active in.
Are you recommending cisco to consider the findings and go for an analysis for a next firmware patch ?
No, I know no one that knows someone that can make such kind of recommendation.
OR they have their own process to follow when identifying bugs those leads to patching ?
I can't confirm nor reject it. If such process exists, it's internal process and we are allowed to know nothing about it. If it exist at all, it seems to be rather random or very low priority process as so many discovered bugs, even those affecting security, are/has been left untouched for months or years.
how to move forward ?
You know I'm not affiliated with Cisco. Thus I can forget claims good for marketing only and I can say just true.
You are on your own, like all of us here. Try to call SMB Support Center, it will not harm, but doesn't wish for solution from them so much. They may help, but according my experience, your issue is so technical and complex, so they will not understood it. You will receive ticket number as the only feedback ever.
Hard to advice something useful. Try other solutions.
- You discovered the PAP2T+SPA3102 works. Who care they are EOL ? Use them.
- Avoid long-line DTMF dialing (e.g. use SIP signalling to deliver called number). It will require a SIP proxy.
- Consider professional converters (but beware that any solution depend on quality and reliability of Internet connection).
- Use ISDN instead of POTS line, if available in Gampaha (I'm not familiar with such region enough).
- Try use a VoIP provider to terminate calls to Sri Lanka. It may not be Sri Lanka's local one. I assume your current price from Malaysia to Sri Lanka is about $0.72 per minute. There are international operators offering calls to Sri Lanka for less than $0.14 per minute.
Note that no advice may help. As I said, you are on yours own. But it's the best I know. Sorry, it's so hard to debug issues remotely, on the other end of known world ;-)
