I think it is impossible that network accepted as IMSI only.
In TS 24.301 220.127.116.11.4, it said:
Depending on the value of the EPS attach result IE received in the ATTACH ACCEPT message, the following different cases can be distinguished:
1) The EPS attach result IE value indicates "combined EPS/IMSI attach": attach for EPS and non-EPS services, or for EPS services and "SMS only" have been successful.
2) The EPS attach result IE value indicates "EPS only": attach for EPS services has been successful but attach for non-EPS services or "SMS only" has failed.
There is just two cases. Furthermore,
in TS 24.301 8.2.1 "EPS attach result" IE is mandatory in attach accept and
in TS 24.301 18.104.22.168, there are only two cases of EPS attach result IE:
0 0 1 EPS only
0 1 0 combined EPS/IMSI attach
You can download TS24.301 here: