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### Description

I followed the page 18~23 of this slide 
[20-Federico.Cabiddu-Kamailio-In-A-Mobile-World](https://www.kamailio.org/events/2015-KamailioWorld/Day2/20-Federico.Cabiddu-Kamailio-In-A-Mobile-World.pdf)
 to implement suspension & push notification, it worked well on mobile devices.

However, callee might have several location records on some occasions, for 
instance, they forcibly quit the app or kill the browser, meaning they don't 
de-register, the location hasn't expired and still exists, but it is stale and 
unreachable, this makes `t_relay` failed (return code is -2 and didn't trigger 
failure_route) since Kamailio `lookup` the stale location rather than the new 
one as they get push notification, below are some part of logs.

```
ERROR: tm [t_fwd.c:484]: prepare_new_uac(): could not build request
ERROR: tm [t_fwd.c:1764]: t_forward_nonack(): failure to add branches
ERROR: <script>: not t_relay by INVITE -2
ERROR: <script>: check status code is 100
```

The rest of Kamailio code triggers `sl_reply_error`, caller gets 500 and 
terminates.

### Expected behavior

When callee has several locations, even if some of them are stale, callee 
should not terminate the call and should find a valid location to ring the 
target.
 
#### Actual observed behavior

Caller finds invalid locations and gets 500 error msg.

#### Debugging Data

What I have tried
1. Run `suspend` when `t_relay` failed.
```
if (!t_relay()) {
    revert_uri();
    route(SUSPEND);
}
```
It doesn't work since `t_continue` still tries to recover the call with the 
stale location.

2. Run `append_branch` before `t_relay`.
```
if (!lookup("location")) {
    send_replay("100", "Trying");
    route(SUSPEND);
} else {
    append_branch($tu);
    t_relay();
    ts_store();
    $sht(vtp=>stored::$rU) = 1;
    xdbg("stored transaction [$T(id_index):$T(id_label)] $fU=> $rU\n");
}
```
This version of caller won't get 500 and terminate, but callee cannot finds the 
resumed call.

3. Use `t_load_contacts` and `t_next_contact`.
```
t_load_contacts();
while (t_next_contacts()) {
    xlog ("==== $ru\n");
}
```
t_next_contacts always return -2 (nothing to do), but I can see the new 
location via command `kamctl ul show`.

#### Log Messages

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#### SIP Traffic

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### Possible Solutions

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### Additional Information

  * **Kamailio Version** - output of `kamailio -v`

```
(paste your output here)
```

* **Operating System**:

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