Main reason WHY some phones doesnt work in some networks is - frequency assigned to network .
For example :
Nokia 5110 (uplink 890-915Mhz , downlink 935-960Mhz ) doesnt work with "new network" because this "New" network uses
expanded "e-gsm" frequencies ( uplink 876-914Mhz , downlink 921-959Mhz ) so phone just doesnt have a channel to "speak" to own network ( sim card tells to communicate to network/network sends ansver via channel that doesnt exist on that phone )..
same goes for Benefon Twin : this phone supports the same "p-gsm"(Primary gsm) as 5110 ( twin`s service manual chapter 1.1 )
Why doesnt it works on 1800 , ihave just a theory ( facts is kinda tricky to find ) :
phone is programmed to register via 900 band primary , but when fails to do ( network doesn have a old "p-gsm" ) ....
other reasons can be configuration of basestations( network ) , sim configs ,etc .