If you mean on Plusnet, see Section 2
Your current gateway
If you mean WBC handover nodes, such as the locations shown in Section 1, there are 20 dotted around the country. WBC-using ISPs use all twenty, WBMC-using ISPs (Plusnet and nearly all the other BT Wholesale ones) use a subset of those. Small ISPs may only use 1.
There are a separate 10 nodes for IPStream Connect, though they are physically at the same place as ten of the WBC ones
The handover at these nodes is via MSILs.