Why should my phone issue an IP address but not an DNS address?

[AC Question]

4.3 phone as wifi AP: laptop set to auto-obtain IP+DNS, obtains IP but not DNS

Hi...I am using a Coolpad 8730L LTE phone (Jelly Bean) to connect to Airtel 4G in India (name of wireless provider is probably not relevant). I can connect without a problem and internet access on the phone works normally.

However, if I activate the wifi access point on the phone, and try to use my Win7 laptop through wifi, I have the following strange problem. If I enter numerical IP addresses on the laptop browser, this works fine, but names do not.

The above is if laptop is set to obtain an IP address and DNS server addresses automatically. If I manually enter a DNS server (e.g. Google DNS) on the laptop, but leave it to auto-obtain IP address, then the laptop works fine.

Incidentally the laptop has no intrinsic problem; it works correctly with other wifi access points.

Why should the phone issue the laptop an IP address but not DNS addresses? It clearly knows the DNS address since name resolution works correctly on the phone.

How can I fix this? Many thanks!