The error is correct:
 
all query columns must be indexed or boolean: column=DisplayName
 
We put a restriction on what filters can be applied as some filters would can internal database performance issues.  Unfortunately, the display name column is not indexed on call handler templates.  I've sent an internal email to the data team to see if we can get that field indexed, but it won't help for current releases.
 
Complete error response pasted below:
 
<ErrorDetails>
<errors>
<code>INVALID_PARAMETER</code>
<message>
all query columns must be indexed or boolean: column=DisplayName
</message>
</errors>
</ErrorDetails>

It appears that the only columns indexed on call handler templates are:  ObjectId, FileName and AfterMessageActionObjectId.  Internally it looks like there is an agreement that we can index the DisplayName column as well, but it won't help for current releases.
 
-Dave