Are there any apps that let you use your cell phone as a modem?

[Moogle!]

In conjunction with a usb to serial adapter?

 

[jafir]

Maybe you could use an acoustic coupler with one of these?

 

[Malc]

Many years ago i used an old mobile / cell phone as a modem, It was dog slow but it worked, IIRC i had to change a setting in the phones setup and my computer recognised it as a modem, It was connected via USB though.

 

[Moogle!]

That seems to depend on the quality of the handset. I've seen it work, but mine is sketchy.

 

[KC9UDX]

In conjunction with a usb to serial adapter?

Certainly. But do any phones that run PalmOS (HP WebOS) work on any modern cell networks?

 

[NeXT]

The last phone I owned and could use as a modem (via serial or PCMCIA up to 9600bps) was a Motorola Timeport that I had to retire when CDMA was shutdown in 2014.
I have not seen a smartphone that could operate as a serial terminal with a USB dongle in years. I've never seen anything remotely close to an app that bridged 3G/LTE/whatever data into an emulated modem. The closest was a smartphone running in hotspot mode and a Wifi-232.

 

[eeguru]

There are several projects (including some by forum members) that add an ESP8266 to a D-sub connector to create a modem emulator over WiFi. You could pair that with a hotspot running on your phone. All add functionality to the ESP8266 to speak Hayes AT command set. Some use a dialed number to IPort alias table. Some allow you to ATDT<ip>:<port> directly.