For INTL dial out, POC needs '00' not '+' prefix

For MO calls of type INTERNATIONAL, we prefix with a '+', but for POC
we need to change this to '00', now people who have intl numbers stored in
their phone books will be able to dial them.

(this was brought to my attention last year on day 4 just before tear down.)

src/mncc.c

@@ -355,7 +355,7 @@ static void continue_mo_call(struct mncc_call_leg *leg)
 	leg->state = MNCC_CC_PROCEEDING;
 
 	if (leg->called.type == GSM340_TYPE_INTERNATIONAL)
-		dest = talloc_asprintf(leg, "+%.32s", leg->called.number);
+		dest = talloc_asprintf(leg, "00%.32s", leg->called.number);
 	else
 		dest = talloc_asprintf(leg, "%.32s", leg->called.number);