GST input on Mobile

Yes u can take input at the time of purchase itself.

On the purchase time you can take ITC

Yes
Itc can be taken at the time of purchase for business.

Yes you can claim full ITC on Mobile Phone which is being used in the course of business. As per the Provision of Section 16(1) of the CGST Act 2017, ITC on Inputs, Input Services and Capital Goods "used in the course of or furtherance of business" can be claimed subject to Blocked Credit U/s 17(5) of the CGST Act 2017.

It depends upon Which business you carried out???

If you deals in exempted supplies then no Input available as per Section 17(3).