Noting that IMTU means In My Traveller Universe - i.e. not the OTU -and, as explicitly stated, I am only sharing what I do IMTU.
Indeed, local taxation of local currency does exist (but is rare given the benefits of the Imperial system) - however, for systems which are part of the Imperium and trade in 'Imperial' CR, it is required that they abide by the Imperial rules - to wit, CR are never taxed.
Since this is MTU the idea that many world-wide comm systems would not be a public service is wrong. Global and system comm networks are associated with Imperial starports, along with stellar and planetary 'weather' reporting. They are always 'public' to the interstellar community, though that does not mean the local populace necessarily is allowed access by the local government nor that comm devices are legal outside a starbase or other Imperial territory.
BTW, in the RW, there are several countries where Satphones are illegal (or there used to be). Also, RW, a Satphone can be tracked (doppler), but not as precisely as a cell phone user. IMTU I presume a (true) GPS type tracking system where the Satphone tells the network where it is very precisely. This has an advantage that it
can be tampered with at the handset (i.e. without hacking the network).
As to lower TLs, obviously if they lack space faring ability and access to such, they do not have Satphones - and also, by definition of not having access, are not be part of the trade system IMTU. In CT hand calculators come at TL7 the same as non-starships, so cell phones are arguably at the same TL anyway.
It has been standard IMTU for starships to have a 'global comm package' which consists of several dozen micro-sats (for LEO) and handheld comms (my term from the '80s, today they are probably called nanosats) for ship to party communications and GPS.
Mostly these exist in my game for metagame reasons - so the party can 'stay connected' and not be bothered with taxes (except when I desire them not to be :devil

. These aspects of MTU date back to circa '83 shortly after the downed KAL incident when Ronald Reagan committed the U.S. to providing GPS to the public - and Satphones were already commonly in use by international shipping. (Since my dad worked at both a shipyard and NASA in years prior, I was quite aware of these tech options.)