Thanks for the reply PhoneBoy
Hopefully should get Wireshark logs fairly soon. Following further investigation today, it seems as though the Mobile Access blade is synced with AD. So when the users switch between mobile device and laptop, the username stays intact, therefore keeping the same IP. This is fine.
The issue of calls remains though. Does the Teams App have to remain open on the laptop in order for the call to go through? It's really odd. Texting via Teams works while the app is closed but not the audio call.
Looking at the firewall policy, I wonder whether a Lync rule is interfering with Teams connectivity (please see attached). The Teams rule is fairly open at the moment. It allows any to a wide range of MS Teams IPs over http, https and UDP_3478-3481.
Assistance would be greatly appreciated.