Just returned from Malaysia. Used the MR Talk app successfully on 1+6 (not supported locally for VoLTE).
Deliberately didn't subscribe to MR roaming, as the app is an SIP VOIP app, so all i need is an internet connection.
Used roaming from eight, simba, and "borrowed" friend's digi line as hotspot, and public/hotel free wifi.
no problems making and receiving calls. the phones were singtel and eight roaming subscribers as well as local Malaysian numbers.
To answer your questions:
1. the app is a VOIP app using SIP.
2. i don't know anyone using this app, so i couldn't verify MR Talk <--> MR Talk connection path
3. if in SG, use VoLTE or other VOIP apps for calling eg Whatsapp.
4. when roaming, forward calls to MR server (one-touch button is provided in Talk app), so that backend will direct incoming calls to app. These calls will be chargeable at 6+ cents/minute (presumably minimum charging block at 1min -- will confirm when i get the bill later)
for unsupported VoLTE phones in SG, leave forwarding on so it's still functional. Will see when the next bill comes whether these (forwarded) local calls are chargeable
5. no need to set MR Talk app as default phone app.
also, no need to change the battery optimisation settings -- i left it at "intelligent" or "optimise"
also, no need to lock the app in background (via recents menu).
It just works.
6. the potential drawback is that it runs on m1 4G network locally. Since this is not my primary phone, I'll find a way to deal with it. Anyway, it appears they have improved coverage, so i get better signal strength and speed at home compared to 2 years ago (when i was on CL).
7. another drawback is can't use shopee to pre-pay, so no discount.
8. speculation purely on my part. since MR Talk is a SIP app, it opens the possibility of being a business customer and getting around the VoLTE issue.
MR runs a VoIP Business Voice Service - hint, hint, hint. read between the lines.
conclusion. i'll probably port over my Simba line to MR, given the limitations of Simba Voice app on my phones, and them stating that they no longer support it.
Sure, i may miss the roaming, but at least my old but still very useable phones will not become ewaste. For roaming, buying a local pSIM/eSIM is still an affordable option, so the loss of Simba roaming won't be a major issue
(Note: Simba Voice app still works, for now)
More travel lined up for the next few months. Will see how the MR Talk app fares but I don't expect much difference.