Well, if you’re only using a set of two radios, it’s probably not even worth the cost of creating a gateway or other methods of ‘over IP’ crosslinking the units. If those radio were bluetooth audio equipped, probably not, you could skip a handful of hurdles as quite a few BT equipped handhelds can function as BT speaker phone units when paired to a mobile phone/tablet. So, post pairing, it’s more on-device routing your connected IP gateway app to use the BT connection for audio and set up vox mode.
But i suspect that’s not on the cards, and retrofit BT adaptors that plug into existing external speaker-mic socketry of the better radios isn’t really worth the cost.
If there are more than a handful of sets, and you are potentially looking at replacements or a bigger rollout of sets in use, then maybe something RoIP/POC based (essentially using 3G/4G/LTE based ‘radios’ which are glorified Android phones mostly) may be the saner route - or more simply, use a digital voice communicator app on Android/IOS phones as that’s essentially the same as the cellular or wifi based RoIP/POC setup in the often overpriced fake radio handsets sold for that as dedicated hardware.
If you have great public outdoor wifi access, the most Android/IOS devices with ‘radio’ communicator DV apps can be switched to Wifi where it’s accessible and fallback to cellular data when it’s not. Same story for ‘network radios’ (the fake ‘radio’ android based hand helds and mobiles).
But you may not even need go to that extent - using existing kit and (where you can legally set up and use where coverage allows also) a mini site repeater is one option open. If the off-site locations are very line of sight but distant, you may find a repeater does the trick ok.
Again, subject to site, license and regional rules limitations, it’s not that hard to turn two old radios into the two halves of a simple repeater (and i do mean simple, as in bare bones and more a proof of principle than really effective) - but that’s overlooking the fact it’ll never be TA certified and probably not legit under an LMR licensed use.
There are examples of basic ‘extender’ type repeaters which just act as an intermediate radio pair so you access it and work simplex communication only the repeater extender acts like a wifi extender. But i’ve no hands on experience of them, and the few budget ones i’ve seen are pretty much trash builds not fit for purpose beyond technically working.
Alternatively, again, but unless you want to venture into TA-certification dubious gear, there are chinese clones of BoxChip S series radio hybrid cellphones - these use a cellular phone with an extra UHF/VHF or dual band radio transceiver and have real ‘walkie talkie’ mode operation like the high grade BoxChip and RFinder units (don’t bother with RFinder for commercial use, you’ll never justify the cost). I have one (of the clones) i use as a UHF transceiver for accessing the local ham repeater when i’m forced outdoors and can’t reasonably take a decent dual band unit with me (the phone hybrid sort of looks like a cellphone with a bigger helical antenna on it for the UHF side). The cheapie clones aint great, TA-dubious at the polite end of being generous (not that it matters for my use), and really only designed for the chinese PDT system use, but are DMR interoperable (they work at least, but only really as Tier 2 items assuming you get proper TA certified examples) and can be used (in principle at least) as analogue FM for PMR446 usage (not necessarily legally).
So having outlined some generic ideas, i’ll hand back the reins to whoever can best advise based on your local and licensing restrictions that go with the actual territory.