I'm trying to confirm that it was just cell interference.
That's most likely. Could be a pager, could be a tablet, but it's definitely a digital transmission of some kind. Doesn't have to be your phone, or even a phone particularly close to you. Whatever it is, it's prodigiously leaking RFI.
I just want to make sure that there isn't something else that I'm doing wrong.
The best weapon we have to fight RFI is balanced interconnects. Good balanced cables with good XLR connectors like the new-ish Neutrik EMC series XLR connectors are your best bet. IIRC, your Sony doesn't use XLR connectors, and that unbalanced cable sitting out there between your CA-9100 and your Sony PCM-M10 can act like an antenna. If this continues to be a problem for you, one path out is to upgrade to equipment that uses XLRs.
Another common path RFI can take to get into your system is through power cables, especially if a wall wart is involved. Battery operation solves this one.