CDMA & GSM or GSM only

There is really no such thing as a "CDMA SIM". Some WCDMA carriers use something like a SIM. To the best of my knowledge, no iPhone supports WCDMA.

It sounds as if your phone is GSM if it gives you that error message. Your friend is using a CDMA carrier but has a GSM SIM in his phone for international roaming.

All iPhones since the 4s, have all had both CDMA and GSM hardware inside.

That does not mean they can magically connect to to both at the same time however.

Some CDMA carriers use GSM sim cards for Data access now. But only on iPhones that support LTE. so iPhone 5 and above.

If you have an iPhone that has service from a GSM carrier, removing the sim card will effectively kill the GSM connection to the carrier, so no service.

CDMA carriers configure the phone to use their network, and generally do not require a sim card, since all connection settings are saved onto the CDMA hardware inside the phone. If a sim card is present in a iPhone setup for a CDMA carrier, removing it will not cause the service to go away, since its not being used.,

If your friend uses a sim card for international service on GSM networks, then he can pull it out when he is in the originating country and the phone will remain connected to his CDMA carrier.

It appears you on the other hand are not connected to a CDMA carrier at all, so removing the ism card from your iPhone cuts the connection to your carrier, and so you have no service.

That doesn't change the facts. The answer is no. You can not use that phone with a CDMA carrier. A CDMA 4S can only be used on the CDMA carrier it was originally sold for. An unlocked GSM iPhone 4S can not be used on any CDMA carrier.

No, if you are using a GSM carrier, then you cannot use a CDMA carrier. It only works the other way.

i.e You iPhone needs to have been sold for a CDMA carrier originally. Otherwise it will only ever use GSM carriers and sim cards.