A normal phone contract is a financial agreement. Networks often run a credit check on new customers to review their credit score.
For SIM only contracts, it depends on the deal you’re going for. If you’re paying for the entire term of the contract in advance, there won’t be a credit check. This usually applies to 30-day contracts, but it’s still worth checking with the network if you’re worried you’ll fail a potential credit check.
If you’re going for a longer-term deal (like a 12 or 24 month contract) they’re more likely to conduct a credit check on you. This is because you won’t be paying upfront.