Either there is an issue with the soft phones, in particular in relation to early media, or your dialplan has answered the incoming call. We need to see the earlier parts of the dialplan, in particular to look for any call of Progress, or Answer, or anything which would implicitly answer the incoming side.
I don’t think you are actually answering, as I don’t think you would get the making progress message if the incoming channel was up.
I think avoiding calling Progress (and of course not answering) will avoid early media reaching the phone. However, if the gateway doesn’t send early media, you will probably here a locally generated ringback even when the network is sending busy in band.
If the phones are triggering on 183 itself, the only tactic I can think of is to use & and a local channel that never answers, in the dialstring.
I’m guessing that FreeSwitch either doesn’t support early media, or fails to forward 183, i.e. this is an interaction between a buggy phone and a more complete implementation of SIP.