new UM card with +1/+44 dual number soon

[inquisitor]

Incoming calls will always work for both numbers, no matter where and through which IMSI you roam.
When you're using the Jersey IMSI, calls (and hopefully SMS) to the US number will be forwarded to the Jersey number.
When you're using the US IMSI, calls (and hopefully SMS) to the Jersey number will be forwarded to the US number.

The fact the US IMSI comes from Immix does not necessarily mean, you'll get the same international roaming capabilites as Immix customers. From what I read, I assume, that the US IMSI will register only on US networks excluding Canada and the Carribean.
If I'm wrong and 44/1 customers do get the same roaming capabilites like Immix customers, that would mean the US IMSI would work also on these networks, which include O2 Ireland, Vodafone Ireland and VOXmobile Luxembourg.

Whether international roaming (i.e. outside the USA) will work with the Immix IMSI is not a technical question, e.g. of the numbering plan, but depends on what UM has decided from the economic point of view. Maybe letting a customer roam on Rogers is cheaper for Jersey Telecom than for Immix - then it would make sense to enable roaming on Rogers for the Jersey IMSI only. Another reason could be the price of outgoing calls - if 44/1 uses direct calling for the US IMSI, there could be enough margin for calls inside the US. But in Canada direct calling could result in higher costs than letting customers roam with the Jersey IMSI and realizing calls through callback.