Having trouble getting calls (in or out) while at retail stores

Then switch to a carrier that has better coverage.

Xfinity does not own and operate their own network. They are an MVNO, piggy backing on a network belonging to Verizon.

If T-Mobile and Verizon both have issues, try AT&T, or ask a friend with AT&T to check their coverage inside the stores and let you know if it's any better.

If AT&T is poor as well, you may just live in a cellular desert. There is no magic fix for lack of coverage.

If the cellular coverage there is poor, yes. If the building materials block radio signals, yes.

Some networks may work better than others. And it's not just stores. I have AT&T. Coverage in my house is marginal, but WiFi calling solves that problem. At work, Verizon is virtually unusable in the building, but AT&T and T-Mobile are great.

Go outside to the parking lot.

Coverage in stores is dependent upon the type of construction, how much metal structure is used, and the available access to outside cell towers. Do you have adequate coverage in other stores?

Probably because the signal in the store is lousy due to the construction and lack of coverage by your cellular carrier.

Why are you telling us?