SDR for listening does not require a license. For transmission, depending on the power and frequency may require a broadcast license.

https://www.reddit.com/r/RTLSDR/comments/dx5sln/do_developer...

Depending on the power of the walkie talkie, it may require a license.

https://www.rcscommunications.com/which-two-way-radios-requi...

> MURS (Multi-Use Radio Service) – Two-way radios programmed to operate within the MURS (Multi-Use Radio Service) are not required to be licensed. They transmit at 2 watts or less and only operate on pre-set frequencies between 151 -154 MHz in the VHF band. MURS radios have a general lack of privacy, a limited coverage area, and frequent channel interference.

> ...

> GMRS (General Mobile Radio Service) – The General Mobile Radio Service (GMRS) is another of the most popular and numerous licenses the FCC granted. GMRS licenses allow for radios to transmit up to 50 watts. GMRS licenses also allow for hand-held, mobile, and repeater devices. The GMRS spectrum has 22 channels that it shares with FRS and an additional 8 repeater channels that are exclusive to GMRS.

> Virtually Every Other Land Mobile Radio (LMR) Device – Virtually all two-way radios beyond the models mentioned above are subject to FCC licensing. In fact, any device that transmits at 4 watts or higher requires coordination (and, thereby, licensing) by the FCC.

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The Flipper is licensed to operate with a particular set of power and frequency ranges. https://flipperzero.one/compliance

For the SDR it is licensed to operate between 304.5 - 321.95; 433.075 - 434.775; and 915.0 - 927.95 MHZ range in the US.

Note that none of those are the cellphone frequency bands.

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https://prplfoundation.org/yes-the-fcc-might-ban-your-operat...

which quotes 2.1033 Application for grant of certification. Paragraph 4(i):

> For devices including modular transmitters which are software defined radios and use software to control the radio or other parameters subject to the Commission’s rules, the description must include details of the equipment’s capabilities for software modification and upgradeability, including all frequency bands, power levels, modulation types, or other modes of operation for which the device is designed to operate, whether or not the device will be initially marketed with all modes enabled. The description must state which parties will be authorized to make software changes (e.g., the grantee, wireless service providers, other authorized parties) and the software controls that are provided to prevent unauthorized parties from enabling different modes of operation. Manufacturers must describe the methods used in the device to secure the software in their application for equipment authorization and must include a high level operational description or flow diagram of the software that controls the radio frequency operating parameters. The applicant must provide an attestation that only permissible modes of operation may be selected by a user.

and 2.1042 Certified modular transmitters. Paragraph (8)(e)

> Manufacturers of any radio including certified modular transmitters which includes a software defined radio must take steps to ensure that only software that has been approved with a particular radio can be loaded into that radio. The software must not allow the installers or end-user to operate the transmitter with operating frequencies, output power, modulation types or other radio frequency parameters outside those that were approved. Manufacturers may use means including, but not limited to the use of a private network that allows only authenticated users to download software, electronic signatures in software or coding in hardware that is decoded by software to verify that new software can be legally loaded into a device to meet these requirements.