One of the default settings in the Android version of ReachUC is to put any current ReachUC call on hold if you receive a call on your Cell Phone number.
This option can be turned off by following these steps:
Launch the ReachUC app
In the top right, tap the 3 dots and select "Settings"
In the settings menu, select "Preferences"
In Preferences, select "Controls"
And in here, select "On GSM call" and then select Do Nothing
This selection will allow the GSM call to your Cell Phone to ring and not put your ReachUC call on hold. The default setting of Put Call on hold usually causes more problems than it solves.