Serial console over GSM – Are there such devices

To save your sanity, don't.

Modems (and fax) over VoIP are extremely sensitive to timing, and usually break over VoIP, it might even seem to work at first, but it's certain to break.

Adding virtual machines just makes the problem worse.

There are "short haul modems" around which can emulate a phone line.

Another option is just to attach the two modems back to back:

http://www.jagshouse.com/modem.html

Maybe this will work? Still not sure why you want to follow this path, but the option is available.

Redirect the Direct Console to a Serial Port Using the vSphere Client

When you use the vSphere Client to redirect the direct console to a serial port, the boot option you set persists after subsequent reboots.

You can redirect the direct console to either of the serial ports com1 or com2.

The serial port must not be in use for serial logging and debugging, or for ESX Shell (tty1Port). Prerequisites

1. Verify that you can access the host from the vSphere Client. The serial port must not already be in use for serial logging and debugging, or for ESX Shell (tty1Port).

2. From the vSphere Client, connect to the vCenter Server and select the host in the inventory.

3. Click the Configuration tab.

4. Under Software, click Advanced Settings.

5. In the left pane, expand the VMkernel listing and select Boot.

6. Make sure that the VMkernel.Boot.logPort and VMkernel.Boot.gdbPort fields are not set to use the com port that you want to redirect the direct console to.

7. Set VMkernel.Boot.tty2Port to the serial port to redirect the direct console to: com1 or com2.

8. Click OK.

Reboot the host.

You can now manage the ESXi host remotely from a console connected to the serial port.

https://pubs.vmware.com/vsphere-50/index.jsp?topic=%2Fcom.vmware.vsphere.install.doc_50%2FGUID-C65306C0-DA37-4F45-8A50-31F8D109BB1D.html