1 phase access when the MS is planning to send less tha 8 RLC blocks – it asks for 1 PDCH only.
2 phase access when the MS is planning to send more than 8 RLC blocks, or when it is planning to use EGPRS, as far as i remember – it sends its multislot configuration so that the PCU can allocate as many PDCH as possible for the MS.
In both cases, it is up to the MS to decide whether to do 1 or 2 phase access.
There are other constraints to do a 1 or 2 phase access, but I forgot them (RLC ack or unack mode, EGPRS modulation, etc etc)
But the principle is the same.
Big transfer = 2 phase
Tiny transfer = 1 phase
Decision’s criteria are hard coded in the MS.