this functionality is not implemented, you might create Customer Feature Request
The main intent of IPSLA operations, that you probably mean when you note there is no access to GW from you side, is to monitor quality of the lines, to simulate the traffic by active elements of the network.
Simulation of call from phone device to phone device is not implemented, the substitute for this functionality are for instance Voip UDP jitter, ICMP path jitter, ICMP path echo.