Question on Revenue

I don’t understand the journal entries below

On 31 January 2017, Terry entered into a contract of a 6-month 4.5G Data and Wifi Plan.

The contract terms are listed as below:
1. The fixed monthly fee is $600 per month; and
2. At the inception stage of the plan, a free Phone is given to Terry for inviting him to join the service plan. The 4G Data Service and the Wifi Service will start on 1 February 2017 and will end on 31 July 2017. Terry pays cash on every month end starting from 28 February 2017.

The stand-alone selling prices of the Phone, the 4G Data Service and the Wifi Service are $2,000, $400 per month and $100 per month respectively. The cost of the X-Phone is $1,100. You should assume the X-Phone; the 4G Data Service and the Wifi Service are three separate performance obligations. Perpetual inventory system is adopted.

Prepare journal entries from contract inception up to 31 March 2017.

31-Jan-17 Dr. Contract asset 1440
Cr. Revenue (Phone) 1440
Dr. Cost of sales 1100
Cr Inventory 1100

1-Feb-17 Dr. Receivable 600
Cr. Contract asset 240
Cr. Contract liability (4G) 288
Cr. Contract liability (Wifi) 72

It is part of the entries.

I would like to know why should use Contract liability? As I know Contract liability occurs because of the customer pays consideration before the obligation to transfer goods or services to a customer. In the case, the 4G Data Service and the Wifi Service will start on 1 February 2017 and Terry pays cash on every month end starting from 28 February 2017. That means the services transferred before receiving cash. Why the answer say it is a Contract liability

Please help, thank you