Question: what is atcf in ims?
Answer: The “ATCF” is short for “Access Transfer Control Function”. As to how it gets a location update, if you mean from the MSC/VLR in a VPLMN (one thing that you could mean by this question), then it would be the I2 reference point, where the ATCF is within the VPLMN (as is, practically, the P-CSCF and ATGW).