My mobile is a Motorola G5 (XT1676) with a dual SIM facility.
I have two SIMs installed. Slot 1 is card on 3 which is essentially a backup, slot 2 is my main provider PLusnet. That card is configured as preferred SIM for Data, Voice Calls and SMS.
If I wish I can configure it, via contacts, to use the 3 card for specified contact numbers. I'd also expect to be able to receive incoming calls to my 3 number should somebody choose to use it.
However, wherever I am in UK the 3 Network shows as voice calls only. I cannot call out even to numbers selected to use 3 Attempts to call to the number receive a 'this number is not currently available' message.
Whilst on holiday in France it worked as expected; Plusnet and 3 both seem to roam by preference on the Orange F network.
I've tried to get help from 3's (offshore) contact centre but lost faith pretty quickly when told issue was that one SIM Slot on my phone only supports 2G networks. When I corrected that, pointing out that swapping cards between slots made difference we went through a 'remove card' check. I'm then told I have to select 'always ask' instead of PLusnet for voice. That of course has no effect either.
Inevitably I get a bit impatient at which point the operative starts talking down to me about 'trying to make me understand'. I gave up before my blood pressure went off scale.
More digging on my part suggests a phone issue. Found this on a forum:
Seems under the software on the G5, whichever SIM card that is NOT set to handle mobile data, tries to default to 2G as preferred network (under Settings / More / Mobile networks) and the "Preferred network type" button is greyed out and inactive.
Presumably Orange F still supports 2 G hence why it worked in France.
...been in exactly the same situation as you with a previous Motorola dual SIM phone.
3, of course, have never supported 2G (hence their name), which is where some of my issues arose. ie. Using the three sim and not selecting it as the data sim just didn't work, but ISTR that it did when roaming, if the roamed network (not native 3) supported 2G.