I was also not able to get a 2year phone contract with Sunrise who was already my service provider. My phone was stolen at Zurichfest 2019 so I went to Sunrise to get a new phone. At the time, they had the Galaxy S10 5G phone only available with a 2 year monthly payment plan and I couldn't buy it in full. It seems any contract that is longer than a yearly period is impossible to get.
I finally hit the two year mark and I'm getting my B permit this month!
I moved to CH in the end of 2012 and have had a non-EU L permit for 2 years.
The only thing I can remember is not being able to get a mobile phone contract from Salt (then Orange CH). They started accepting L permits for contracts in 2013 though but I was pretty happy with their prepaid number anyway back then.
I've heard a lot about people on L permits being refused credit cards but mine got approved in my first month in CH with a standard 5k CHF limit with absolutely no questions asked.
Seeing as an L permit is a short term permit of up to one year and there being a real possibility that you'll leave the country after the permit expires you'll likely have a problem with subscriptions that last more than one year. e.g. phones, internet etc but there's usually a solution to be found.
If the permit is based on employment and your job is for less than one year the chance of getting an L permit (rather than a B) is as close to 100% as you can get unless your employer makes a less than accurate application.
It was the UBS branch at Bahnhofstrasse. I think they were free for the first two years for new clients back then, and they also took on US people (not an issue for me but plenty of them working for my employer so this branch was the default recommendation).
Croatian nationals have quotas, and no matter if contract is unlimited or limited, we get L as the first one by default, from all what I've seen/heard so far.That's why I said it's 99% we'll get L.
But I definitely understand how once you have L, all other services rely only on common distinction L = short term, B = long term.