I had to add the following code:
int[] tId = new int[1];
int status;
pjsua.transport_config_default(cfgTrasport);
cfgTrasport.setPort(5060);
status = pjsua.transport_create(pjsip_transport_type_e.PJSIP_TRANSPORT_UDP, cfgTrasport, tId);
if (status != pjsuaConstants.PJ_SUCCESS) {
Log.e("pjsua.transport_create returned status="+status);
}