Create phone call using web services problem

Hello,

I'm trying to create a phonecall and assign the from and to attributes:

Entity phone = new Entity("phone");
phone.attribute["subject"] = "hello";
phone.attribute["to"] = new EntityReference("systemuser", new Guid("xxx"));
phone.attribute["from"] = new EntityReference("systemuser", new Guid("xxx"));
phone.attribute["regardingobjectid"] = new EntityReference("account", new Guid("xxx"));

After added the record, regardingobjectid is completed but not FROM and TO.

Thank you in advance for your support.
Regards,