Arabic text/SMS speak is a thing too, see https://en.wikipedia.org/wiki/Arabic_chat_alphabet; it's invented by necessity because sms only supports latin (ascii?) characters.

SMS can use Unicode, but it uses UCS2[1] so you get less than half the characters you get with the bitpacked 7 bit SMS character set. The 7 bit is mostly Latin, and uppercase Greek (using latin characters when identical). There are also some 7 bit shift-tables for some languages, including some Arabic for Urdu (would this cover other Arabic using languages?), but I'm not sure if anything use these tables instead of just going for Unicode as soon as it leaves the basic set.

I guess there might be input issues as well, do cheap feature phones have an Arabic input mode?

[1] Yes, UCS2 accord to the specs. I suspect modern systems ignore this bit of the standard and use UTF16.

Urdu extends Persian, not Arabic. Those two are very, very close but not identical IIRC (it's been 20 years since I studied Arabic). Disregarding pronunciation differences of the same character, there are a few different characters.

To clarify, the scripts not the languages are close.

afaik it was because of people living outside Arab countries who didn't have Arabic keyboards or didn't like switching the languages too much. It was always possible to send SMS in Arabic, atleast for the past 15 years.