Cobra has used 2 different methods to provide a call function for 2-way radios:

ANALOG: The vast majority of FRS and GMRS radios use an analog call signal.  It is an internally-generated sound that gets transmitted.  The receiving radio cannot tell the difference between an analog call signal and your voice -- both are just sounds that get received and you hear them from the speaker.  When you hear the analog call signal YOU know that it is a call signal, but to the radio it is just another sound like your voice.  It cannot tell the difference.  

DIGITAL: Some Cobra radios have a feature called VibrAlert, where they will vibrate like a pager.  In order to vibrate, the radio must be able to identify that an incoming signal is truly a call signal.  An analog call signal won't work, because it is just another sound like your voice (as described above).  In order to implement the vibrate feature, Cobra uses a digital call signal so that the receiving radio can identify it as a true call signal.

Unfortunately the CALL signal is not 100% compatible between analog and digital models.  In addition, Cobra has used 2 different digital call signals (due to advancements in technology).  So not all digital models are compatible between themselves.  Here are the "rules":

GROUP 1 - DIGITAL CALL SIGNAL (1st Generation)

The call function is fully compatible between these models:

FRS250, FRS300, FRS305, FRS310, FRS315

GROUP 2 - DIGITAL CALL SIGNAL (2nd Generation)

The call function is fully compatible between these models:

PR950, PR1050, PR1100, PR3000, PR4000

GROUP 3 - ANALOG CALL SIGNAL

All other models use an analog call signal.

If GROUP 3 calls GROUP 1 or GROUP 2, they will ring normally but they WILL NOT VIBRATE.

If GROUP 1 or GROUP 2 calls any other group, you will hear a weird buzzing sound.  But if you are using subcodes, then you will hear nothing.

PLEASE NOTE that normal communications are not affected and will work properly.  Only the call signal is affected.