It appears that I am not the only Psion user to experience my Psion 5mx locking me out during a connection to the internet, via a Nokia 6100 mobile phone in my case. The forum on PDAStreet has a very active thread dedicated to this very problem.
A cursory read through the nine (9) pages of chat suggest that one of three options may fix it: First, update the TCP/IP Stack – files are posted; Second, use an application called Sink to establish the connection; Third, use an ISP which is shown not to cause these freeze-ups.
Once I have investigated this further I’ll update my Psion Modem page and post the appropriate files.