sentences:  If the A-switch group contained many busy PBX lines, then the number of customers might be reduced to 75.The earlier (Mark I and Mark II – the differences being slight) exchanges could handle up to 2,000 customers.Later on, Mark III TXE2s were able to handle up to 4,000 customers and on these exchanges, where the average calling rate was sufficiently low, up to 250 customers would be in an A-switch group, still with access to just 25 A-B trunks.