esn# on iphone confusion

electronic serial number

In you case, can be one of three things

IMEI: at the keypad, type *#06#
IMSI: On the SIM

Or have a look at http://www.tiaonline.org/standards/resources/esn/codes.cfm (apple aren't on there)

I suspect it's either IMEI or serial number in your case

On an iPhone, that is an IMEI number. If that is what your work needs, that is the number.

You can also view this on an iPhone via the menu Settings->General->About and scroll down to IMEI.

GSM phones have an IMEI (ATT, T-Mobile and most all other global carriers - for example)
CDMA phones have an ESN (Sprint, Verizon - for example)

Further to what Len said,

where you dial numbers to make a call, type *#06# here

On an iPhone, that is an IMEI number. If that is what your work needs, that is the number.

You can also view this on an iPhone via the menu Settings->General->About and scroll down to IMEI.