Just would like to share about Simba roaming... My own personal experience; not hear say.
I had used Simba roaming in JB. Fast and good. As far as I know, there had been very few, or close to none, reported problem of roaming in Malaysia.
Just came back from a one-month trip to Hong Kong. Before that i read loads of conflicting feedback. I even personally helped two SG friends troubleshoot Simba roaming in HK. One used iphone while the other a Samsung (don't know what model). Both had roamed in different parts of Malaysia using same phones before going HK. All no problem. But in HK, both can't even get any connection.
So while I am on CMLink, I brought a Simba sim for fun for my HK trip.
First day whole day i have tried to connect (manually) but no telco can connect. Disappointed. Was about to give up. Second day, it miraculously managed to connect to the CMHK 4G. Note that my CMLink was also connecting to the same CMHK 4G network; interesting. So I happily started using Simba roaming in HK!!! Also told my 2 friends: see I can roam; it's proven that it's your (phone) problem.
However, next day I started to realize some apps seemed to be not running well, namely, WeChat, HKemobility, Myobservatory, CityMapper, 中原地圖, 八達通 ... But the data seems to be transmitting!!!! Tested the data upload/download speed, the speed was like 0.001... the network was still on CMHK 4G. Then I switched the data connection to my CMLink. Interestingly, all these apps were working and the network was the same CMHK 4G!!!!!!!
Though the Simba roaming connection was there (4-5 bars) but the speed was just too slow to be useful.
Interestingly in another forum, somebody was saying he had no problem with roaming in HK and he could run some of the apps I mentioned earlier.
Hence the observation/conclusion is, the roaming in HK seems to be highly phone dependent. The result could range from no connection at all, to slow unusable data speed, to no problem. In short, it's unreliable and not dependable.
Similar results were seen in Sydney and Melbourne.
Therefore, i will not use Simba as the main line, especially when traveling. Even it works in one country, it doesn't mean it will work in the others. Imagine what will happen when your main line was completely off (meaning, no phone call, no sms, no otp) for those days when you are overseas? The worst part is, you won't know whether it's going to work until you get there; you can't pre-test anything.
Other telcos or mvno are not that picky on specific phones. So, good luck.