It can be done if you follow a few rules.

If using the older 27Mhz CB style rigs you will have to switch out and isolate virtually every line as you switch from RX to TX.

In the old days before I moved up to Ham Radio I used to do 1 Watt EIRP Packet Radio under the LIPD (Low Interference Potential Device class license) on the in between channels in the 27Mhz band. At the time I simply used a multi pole relay.

Worked quiet well. We even managed to get 1200 baud connections on SSB over quiet long distances by unlocking the clarifier and aligning it in frequency with the TX frequency you could then track up and down a little on both RX and TX to mesh with the rig at the other end.

For phone patch you will also need a COS line which normally won't be available from the standard CB connections. To get this you will have to tap off the squelch. You could also use VOX but that's pretty horrible.

Automatic phone patch units are quiet expensive generally.
A nice open source option instead is to use a Linux box running Asterisk with the extra USB_Radio module and either a home brew modified USB dongle or you can buy them commercially for about US$100 + post. If you DIY the parts are less then $20.

Asterisk can patch you into the phone network via a voip provider and there are also some very nice public radio networks you can join like PTT_LINK.

We are currently setting up a PTT_LINK node here on UHF CB using a TX4200 and variable inversion board to limit the yahoo factor.

-- CB the Future in Australia --
On another note spread the word. ACMA is planning to make all current 40 channel UHF CB radio's illegal to use by 5 years time.
They are planning on going to a 80 channel system.
While expanding to 80 channels is a good thing. Banning the older equipment is a terrible hit to the hip pocket for most people.
And they still haven't considered more important things like opening up the emissions type to digital modes like digital voice and packet radio.
Or allowing 25 Watts power legally. Commercial radio's operate 25 Watts with no qualifications so not allowing CB'rs to do the same is just discrimination.
.-.-.