If your not then are you putting the mifi sim into the phone? if so watch out 3 will detect and disconnect it as it's a breach of the rules.
Data SIMs into phones should be okay, as they're not unlimited - its the unlimited phone SIMs into dongles and MiFi's that they have issues with.
I've had PAYG data SIMs, in various phones in the past, no problems using the phone as a WiFi hotspot, no real difference than the MiFi except phone did later network standard.
James BT Infinity 2
19/09/2012 - Speeds 49 / 8.2 Mbps - Sync 53 / 9.5 Mbps @ 470m
Huawei modem -> RT-N66U -> Switch -> PC/Mac/Linux/NAS/Phone/TV - last speedtest
13 years of broadband - 1999 ntl:(512k/1M)/BTbusiness(2M)/Metronet(2M)/Bulldog(8M/16M)/BE(19M/16M)/BT FTTC(46M)