Crosslinking, essentially link the audio lines of a transceiver to a networked device for IP linking/gateway for Echolink etc isn’t that hard - it’s really no more complex than linking two vox enabled radios via audio line crossover to essentially connect any two radios in any combo of operating mode and bands/allocation on a technical level.
If taking the simplest approach, vox controlled operation, it’s more a case of linking the audio lines as required, setting vox trigger tolerances, and preferably use DCS/CTSS (analogue) or any combo of digital mode radios.
You really do need an effective means, such as access tones, to stop every stray signal received or stray induced sig on the audio link on the respective inputs triggering the link.
So you’ll be wanting to wind your audio cables through ferrite cores and where you can, use good quality coax for each individual audio line, and throw those cheapo audio cables you were going to chop and butcher in the discard pile. In fact, if they were the usual PC interconnect or extensions for 3.5mm audio socketry, about the only bit worth saving are sockets and plugs with an inch or so of existing cable as those will be a breakout means.
I know that sounds harsh, and many people question it harshly, but I use good RF coax (same stuff I use for satellite interconnects) on HiFi (in pairs) and just about any audio application where you’d want a decent degree of shielding, and let’s face it - at SHF where such cable normally gets used, you want super high grade screening and ultra low SIG loss through attenuation. Sure, you’ll never see much attenuation on an interconnect (RF or other) of a few inches to a foot length, but it’s not expensive and you could literally buy the cheapest of that grade and for audio you’ll be really ahead of the game.
Secondly, and this really is where the YT videos and most articles gloss over or omit entirely, you want to set up your connection properly.
By that, I mean get things matched for maximum efficiency. Whilst you can take your headphone/speaker output from the radio linking your radio to your crosslinked mic input lines, and given the input on most modern radios and PC and phone/tablet mics are about 10x the impedance of the source output (as is often the case of inputs on recorders), you can consider impedance matching but it’ll be a negligible return. But what you need to beware of is - mic inputs are intended for micro and millivolts levels (depending on equipment) where your headphone outputs are peaking around 1V or 1.5V (usually about 1V which is about the same as domestic line level). So whilst you can send headphone/earphone audio to a line input without usually overloading it, you will literally swamp a mic input.
So, uncorrelated, you’ll be dynamically compressing the fed audio well into clipping levels. Yes you can turn the output down at Vol control, but by the time you hit a low enough level, you’ll be creating a ■■■■ poor S/N ratio link.
So in your output -> input line, you’ll be wanting an attenuator that’ll reduce the level. If you want to try impedance matching, that can be done in one combined matching circuit that can fit with an XLR shell.
Ditto the above when you feed line level networked device audio in return to your radio’s mic input as the same imbalance occurs. You’ll also want blocking capacitors at the mic socket end to remove the DC phantom source that most electret/condensor inputs provide to what’s normally attached to power the normally present electret/condensor mic insert.
So in short - whether you are fabricating a radio-radio crossover for cross band linking or an improvised repeater or radio to networked audio IP link, you’ll want to at least ensure your levels are not distorting at output and not distorting at respective inputs. A minimum of 3dB down from max input level (based in a full scale reference tone) is a good starting point.
Returning to the tone access suggestion - tone controlled squelch use on radios is probably the best analogue precaution vs accidental triggers by unwanted access.
On digital equivs, you can use an appropriate colour code and encryption if needed to restrict access and unwanted accidental triggers.
At the IP audio output to radio mic input, you may want to consider creating some kind of audio gate to ensure that silence hits the vox control on the radio’s mic circuit - again, this can be achieved many ways, but a simple PC/Pi/Mac solution is to use a software audio intermediate to pass the IP client software’s audio out to, use a software audio gate, pass the gates out to your physical audio output that feeds the mic input on radio.
Now some radios have a discrete set of audio lines and PTT and data lines intended for computer control and data modes on FM (FSK data), if your radio has these, exploit those instead as the logic level will probably be 3-5v on PTT so a simple circuit can trigger it smoothly if you err towards conventional PTT driving. But notably, the audio lines will be line level or close enough as they are often intended to be used by an expensive proprietary combo data-audio-ptt interface where there was intentionally provided design for such use. Test and see, if such animals exist on your linking radio - if they do, it makes things a ■■■■ sight less razor edged and bodge engineering to make up a crosslink with.
Either using a properly matched/level attenuated audio linkup or an appropriate use of available accessory/cat type linking socketry is part of why my own setups work smoothly and virtually hit the ground running bar a few fine tuning adjusts.
Yes it’s an obsessive minded approach, but you’ll find every good engineered solution on any level was due to an obsessive minded attention to detail and subsequently engineering the method to work with a finely tuned degree of stability.
If a mere tuppence bit of lashup wiring was the sum and whole of necessity, most crosslinking efforts on any kind of system would have been definitely not as good in end result terms as what you find in any good solution.
Of course, be it crosslinking telephony to radio, radio to radio or any RF linked setup that’s two-way, before you embark on it, ensure the license grade you have for the radio covers gateways/repeater usage (most low-rung tiers/levels don’t) and in many parts of the world, even where no mention is made of such, you may still need a permit of variation to the license where it’s possible to obtain - what we know as an NOV in the UK, a Notice Of Variation that provides limited exemptions or permission for non-standard User Service use.
In some countries, it’s a criminal offense for civilians outside of law enforcement and government agencies to crosslink/patch radio equipment, as there are grounds where it can be interpreted as illegal surveillance or usable for such or for subversive bypass communication, so be cautious at worst, at best get yourself fully compliant with whatever requirements are necessary for your intended link.