How to set the 'sender' field value on phonecall entity in plugin on Pre-Operation state

I do it like this, but it can't be saved. why & how to deal with it ?

Entity from = new Entity("activityparty");
from["partyid"] = new EntityReference("contact", cntid);
EntityCollection froms = new EntityCollection(new List<Entity>() { from });
entity["from"] = froms;