Sim not valid

Hello, while installing a new SIM on an iphone 6s,

the activation process kicks in and the message SIM not Valid shows blocking access to any other app.

unless i remove the SIM.

the Error message reads the following:

'SIM not valid. The SIM card that you currently have installed in this iPhone is from a carrier that is not supported under the activation policy that is currently assigned by the activation server. This is not a hardware issue with the iPhone. Please insert another SIM card from a supported carrier or request that this be phone be unlocked to your carrier. Please contact Apple for more information'.

Facts:

1 - I tried the same SIM on my iphone XS MAX , it worked fine

2- Contacted the carrier, nothing is blocked on their end, actually the same phone was used on a different MSISDN from the same carrier

3 - the Carrier also confirmed that the iphone 6s is shown in their systems as connected to the new SIM/MSISDN

3-i tried another SIM on the iphone 6s and same SIM not valid error pops out.

any help is appreciated !

thanks

Pascal