Iphone 13 contract without tariff

Hey,

I’m looking to get the iPhone 13 on contract but without the tariff since the are i live in does not have good connection for o2. but the pay monthly option for me is very enticing.

I would like to know if the phone is locked to only O2 sim cards or is it possible to use other sims as well. I would also like to know any further details about this contract and anything hidden. 

Basically is contract is for the iPhone 13 and i have to pay 36 monthly with a one time payment of 25 euros right?

Thanks in advance for helping me out!