Multi-Access (MA) connectivity is a native 5G
feature that enables a User Equipment (UE) to simultaneously
access the 5G Core (5GC) through multiple heterogeneous
access networks. For military applications, this capability can
improve resilience to communication disruption and eavesdropping by leveraging additional access networks such as tactical
radios. This paper presents, to the best of our knowledge,
the first open-source standard-compliant implementation of
Multi-Access (MA) connectivity on Commercial Off-The-Shelf
(COTS) Android smartphones. Unlike previous laboratory-based
implementations, the proposed implementation addresses key
deployment challenges associated with smartphone platforms,
including Universal Subscriber Identity Module (USIM)-based
authentication and secure-element access from the application
processor. Experimental results demonstrate successful simultaneous connectivity to the 5GC through both over-the-air 5G
New Radio (NR) and L3Harris AN/PRC-163 tactical radios. The
results demonstrate the potential of MA connectivity for resilient
communications on COTS smartphones.