It is just the first 16 bytes.  The nonce as passed from the HSS is a concatenation of a 16 octet random value and a 16 octet sequence number.  The S-CSCF can either pass this combination as the nonce to the UE, or it can append its own data if it wants. In this resync case, the S-CSCF must pass the random value back to the HSS (hence the first 16 octets) plus the 14 octet auts sequence number passed back by the UE.  I've added a reference to table 6.3.3 of TS 29.228 which is the best description of this.