Accessing Call activity using JQuery on TO and FROM

How can I access "call activities" using jquery using filter on 'to' and 'from'. Whenever I use filters for these two, it says bad request (Not found 'to and 'from'), and when I remove it, the query runs successfully.

https://****.crm5.dynamics.com/XRMServices/2011/OrganizationData.svc/PhoneCallSet?$select=Subject

runs successfully but,

https://****.crm5.dynamics.com/XRMServices/2011/OrganizationData.svc/PhoneCallSet?$select=Subject,to,from

doesn't go well and returns nothing with error.

Thanks

Arun