The MAX BR1 only has a single modem so there is no way to route specific traffic over a specific SIM card. The secondary SIM card is for failover and redundancy. Thanks.
The BR1 has two SIM slots but only one modem. This means that only one SIM card can be used at a time.
If one SIM card fails and WAN or WIFI WAN is available, the failover can happen pretty quickly but a new WAN IP address will be used so VoIP traffic will probably need to redial their calls.
If fail over is between the two SIM cards, it can take 20 - 90 seconds for the other SIM to connect to the cellular network as the two SIMs are using the same modem.
For what you describe, please consider a MAX Transit Duo for mobile situation or Balance ONE CORE for fixed lines.