1)> How can this phone act as a "COMPLETE NODE" of BTC? Is this really true? Could you throw your whole node on a Linux computer and, instead, use the HTC phone?
A smartphone is just a less powerful computer that fits in your pocket. For a computer to be a complete node, you only need to run the complete node software. The phones can already do this, and it seems that HTC is simply pre-installing an application to run a full node on this phone.
In fact, right now you can run a complete node on a phone using the Android application ABCore.
-First of all, I have difficulty understanding how you will store the hundreds of GB of data …
Through the use of a high capacity MicroSD card. The phone will have a MicroSD card slot, and MicroSD cards can already have capacities of up to 512 GB. Alternatively, the node could be a pruning node, so it does not actually store the entire blockchain history. Deleted nodes are still complete nodes, they simply do not serve historical blocks to other nodes.
-I'm thinking about the mining activity. How can this small piece of phone chip do mining if it can not achieve high hash rates?
The complete nodes are not necessarily miners. You can have a complete node without mining or produce any block at all.
2) Is a full-node telephone useful and has a positive impact on the btc network in general (what are the positive impacts)?
Yes. Any complete node is useful for the network. The complete nodes validate and retransmit all the blocks and transactions they receive. More complete nodes means that more people are verifying that the blocks and transactions are correct. More complete nodes in general help make Bitcoin more robust and decentralized.