The core of the database must be the account, also the customers. They are giving the money for the company, there isn't any other possibility. Logically everything should reference some of an account.
Second, the users authentifies themselves for the network of the company by their sim ids. Every sim card should have exactly one account (which will pay for its calls), but an account can have multiple sim cards. Thus it is a many-to-one relation.
IMEI id-s are ids of the phones. The mobile company knows them, because this numbers are communicated on every logon, but normally the customers will be identified - and their contract is about - their sim card numbers.
The IMEI Id-s aren't really existing in this database, altought they can be (and mostly, are) logged by the cellphone company.
In this this case, a login of a cellphone is the contacting entity between the sim card id and the imei id.
Thus the relation diagram is the following:
account <--- simcard <---> phone_number <--- login ---> imei_id
I extended the relation diagram with the phone number, although in this last I am not sure. In normal cases it should have a 1:1 relation with the sim cards.