Here you can see how to make a "phone patch" on a VoIP network using 2 SDRs (minimum one with TX capability).
What is a phone patch?
Shortly described, it's the connection between a phone system and radios. It can help if there's no mobile coverage in cases of emergency, or for local needs inside a large private area. I only know of some professional ones being used in Alaska, where there's no mobile coverage.
The exact equipment:
1 HackRF (from Aliexpress)
1 RTL-SDR (from Amazon)
2 radios (Quansheng and radtel, but it works with any 2-way radio)
SDRangel on PC (you need a decent CPU for it to run smoothly, mine has a maximum of 3.30GHz and it runs at 80%)
**DISCLAIMER**
Please ensure compliance with local regulations before attempting this. Avoid interfering with any transmissions. This experiment is conducted on licensed bands with very low power to adhere to legal standards. Always operate responsibly and within the law.