I think this will be difficult anyway, as in this scenario, both the mobile phone and the peer (a local PBX extension for example) sit behind (different) NAT. So you could use SIP/TCP and do a port forwarding to the PBX (which is behind the NAT too), which will probably work. However, the RTP media channel will work only if your SIP phone on the mobile supports ICE/STUN, which i doubt it does (I have not yet seen such product before but I might of course be wrong).
If the RTP does not work due to this issue, I would recommend to use a frontend PBX with session border objects and you need to turn on media relay on the object for the SIP mobile user. That could work.
I have never done such a setup though, so these thoughts are pure paper-ware


Christoph