G’day,
I am new to the backend of voip but here is my problem
Have set up freepbx working fantastically, can make internal calls, place on hold, transfer, voicemail the whole lot, even record everything.
Next step Ive set up an external trunk. Yep works great dial 0 for outside line and bang everything works great, can call inbound as well and ringgroups activate to call the correct phones.
Heres my dilemma, if I ring from my mobile, I cannot transfer the call anywhere without it dropping, this includes if it transfers from ringing ring group 600, to voicemail (that is if it doesn’t get answered).
The mobile disconnects
I have also noticed that some things don’t work 100% like on hold music over the mobile. And if I have a greeting when you ring followed by a ring tone there is no sound what so ever, it rings but I just hear silence on the mobile
I have forwarded all the RTP media ports etc.
Help please? Much appreciated 
You have a NAT problem with your network. More than likely your router is not smart enough to deal with all the translations.