function startCall(uint timestamp, uint8 _v, bytes32 _r, bytes32 _s) public {
    
    address recipient = msg.sender;
    bytes32 callHash = keccak256('Experty.io startCall:', recipient, timestamp);
    address caller = ecrecover(callHash, _v, _r, _s);

    
    require(activeCall[caller] == 0x0);

    
    activeCall[caller] = callHash;
    recipientsMap[callHash] = recipient;

    
    
    endCallRequestDate[caller] = 0;
  }