I seem to recall that some (like his calls with Putin) but not all of his calls with foreign heads of state were kept on the secret server. We'll never know for sure. The White House is not cooperating in the investigation.
Which is his right and anyone else for that matter.
Anyone and everyone has the right to refuse a congressional subpoena. Now, of course, congress can then take that person to court and the court can force that person to testify, and if he refuses afterwards, he will be guilty of obstruction. But, until the refusal is challenged in the courts and the courts says that person must testify, there is nothing illegal about not testifying.
This is where the Dems are going to shoot themselves in the foot, btw.
Schiff has not yet challenged any refusal in the courts, so Trump can not be guilty of obstruction. However, it seems that the Dems are going to charge him with obstruction anyway for this act. Politically this will be fine until Biden is called to testify by the Senate and challenges the subpoena. Then the Republicans can claim Biden is obstructing justice under the same logic the Dems are using for Trump.
The Dems then have three choices. First, they can say Biden is not obstructing, just practicing his right to challenge the subpoena. But by doing so, it will negate their case that Trump is guilty of obstruction. Second, they could save face and agree that Biden is guilty (although unlikely). If they do, this will then create a plausible reason for the request for the investigation. Or third, they can skirt the issue and give non-answers, such as "we think Biden should perhaps testify to clear things ups," not commenting on his guilt or innocents. This though will leave the Republican talking point of his guilt of obstruction the only talking point out there. With a little more time, this too will be massaged to create a plausible reason for the requested investigation.
Like I said before, impeachment is a loosing play. Censure would split the Republican party and created an electoral mess for them. Impeachment though is splitting the Dems and will create a mess for those in purple districts.