"of the sort that may be used in cell phone towers" - I had no idea this was a thing, so I looked it up. Here's a really good overview of the state of the art in 2004, which is already pretty amazing, and obviously things have advanced a lot since then. There's some serious science and engineering involved:

http://www.suptech.com/pdf_publications/2004_superconducting...

Highlights:

"An advantage of superconductors is that their low-loss properties enable the fabrication of a variety of microwave components in a far more compact geometry than is practical using conventional materials. ... bulky cavity resonators and dielectric resonator structures can be replaced with compact microstrip designs fabricated on wafer substrates."

"The difficult requirements for successful growth of epitaxial YBCO films are the need for high substrate temperature (typically 700–800 C) and high oxygen ambient pressure. To simultaneously achieve these conditions during a thermal evaporation process, a unique heater concept was developed ... a rotating substrate platter that serves as a partial vacuum seal for a heated chamber held at elevated oxygen pressure. The platter fills the open end of the chamber, leaving only a very narrow (less than 0.5 mm) slit. This arrangement results in a pressure drop of three orders of magnitude between the oxygen-rich chamber and the rest of the vacuum system. Substrates spend the bulk of their time in the chamber, where they are heated and exposed to oxygen. As the platter rotates, the substrates exit the chamber and are exposed to the flux from evaporation sources for the three metallic constituents of YBCO. As long as oxygen is supplied to the deposited metal atoms within a few tenths of a second, they can form the correct phase of the material. Rotating the platter at a few hundred revolutions per minute accomplishes this in the system."

"...wireless customers expect (and require) long, maintenance-free lifetimes. Thus, one does not have the luxury of routine maintenance such as periodically evacuating dewars or recharging helium in coolers that might be acceptable in other application areas. The cryocooler and dewar, as well as all mechanical parts, must be designed to last for more than 40 000 h of operation, or approximately five years. ... Reducing the possibility of leakage requires that the dewar be permanently welded shut .. all feedthroughs must have leak rates of less than 1^10 cm /s He. This far exceeds the levels of hermeticity typically defined for the electronics industry and, thus, often requires custom solutions."

Page 8: photo of a 2002 high-temperature-semiconductor filter, including cryocooler and coolant dewar, showing that it's not that much bigger than a regular rackmount server!

"The practical benefits of HTS filter systems are regularly seen in field trials. Trials usually involve two weeks of network statistics, before and after installing a SuperFilter... Typically the base station receiver
noise figure is lowered by 3 dB by the installation of the HTS filter system. ... Dropped calls were reduced from an average of 3.0% to 1.7% overall. The range observed was 18.7% to 63.6% reduction over all channels and sectors. The adjacent sectors, without SuperFilters, went from an average of 2.0% to 1.7% dropped call rate. Ineffective attempts went from 3.3% to 2.5%, a 26% average reduction."