The time slots in the E1/T1 is 64kbps. But the entire E1/T1 have different amount of time slots.
So the total rate is quite different.
The time slots are built up by samples of 8bits(ETSI E0) and 8 or 7 bits (ANSI DS0). Sampling rate at 8kHz gives time slot rate of 64kbps or 56kbps.
A ANSI standard phone call and the ETSI phone call in each time slot are not compliant in coding and need to be converted as well.
But your problem is more related to E1/T1 level, so the DS0 or E0 (time slot)level does not matter for the E1 to STM-1 mux.
Further on, a ETSI STM-1 and a ANSI SONET (the 155Mbps signals) are not compatible in all aspects either.
Especially when it comes to E1 or T1 levels whitin the 155Mbps stream.