Let's take a look at keeping your data up while on a call.Does this happen often or has it just happened one time or a few times? Go ahead and perform a restart for your S9. Power it down for thirty seconds before turning it back on. This resolves most connection or settings issues. If the S9 continues to lose its data connection while on a call, run the device through AT&T's Troubleshoot & Resolve Tool. Just sign into your myAT&T account, click on the preceding link, go into Select your issue>Service/network issue>Internet and follow the steps provided. Let us know if this helps.
James, AT&T Community Specialist
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*I am an AT&T employee, and the postings on this site are my own and don't necessarily represent AT&T's position, strategies or opinions.