Hello everyone! Sorin, you inserted SIM card to the 1800MHz slot and phone says "No SIM card"? I've had this same problem on other old phones... I'm not sure if this helps but listen (uh, read) for a second:
I think the problem could be incompatible SIM card voltage. In the 90's SIM card voltage was 5 volts first, then 3 volt cards started to slowly appear at the end of 90's, and finally 1.8 volts many years after that. SIM cards usually support 3 and 1.8 volts, but all SIM cards doesn't support 5 volts which is required by many old phones.
Some phones don't support all voltages either: Nokia 8210 supports 5 and 3 volt SIM cards according to Nokia's service manual. However, Benefon Twin seems to support only 5 volt cards: I downloaded the leaflet datasheet for Twin and it doesn't say "3V/5V SIM support." Later Benefon phones, like Q and Esc, has the support mentioned on their datasheets that so I presume older Benefons supports only 5 volt cards. I guess 3 volt SIM cards weren't popular when Twin was designed.
I'm suggesting that Benefon Twin gives only 5 volts, but your SIM card wants 3 or 1.8 volts. No common voltage = SIM card can't operate = no SIM card found. Your phone could be 100% working, I hope, but the SIM card is incompatible. Try to find another, possibly older, SIM card!