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Hi Angel_UMi.
I did try just using Sim A alone. Same result - calls straight to voicemail.
I have figured that the problem is related to the network type. When Sim A is on 4G, it will not recieve calls. If I set it to 3G or less, calls will come through. Explains why when the preferred mobile data is set to Sim B, the problem goes away; since Sim A automatically goes to 2G or less when Sim B is the preferred service.
I re-checked the service frequency bands of Sim A (4G operates in 700MHz, 900MHz, 1800MHz and 2100MHz) against the capability of the UMi Iron Pro, and it is compatible.
I have also tried Sim A (with 4G) in other phones, and it works without issue (calls come through).
So not sure why Sim A has this problem with the Umi Iron Pro, and not Sim B.
Hope this helps in finding a solution, as I want to use 4G. Does this give any clues as to what the problem is?
Thank you for your help.
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