Maybe it is not an OS issue but phone that is “calling home” :). I would create iptables rules to see what is transfered and where:
ptables -N LOG_AND_DROP
iptables -A OUTPUT -o rmnet_data1 -j LOG_AND_DROP
iptables -A LOG_AND_DROP -m limit --limit 1/s -j LOG --log-prefix "Dropped: "
iptables -A LOG_AND_DROP -j DROP
and the same for ipv6 if used. Should it is SFOS issue iptables will help.