According to Biden Imran Khan is an unrelated person and nothing more then the mayor of Islamabad, So he does not call him because of his status.
here's my opinion.
The rivalry between the moderates and the ultra conservative in Taliban ranks will be even more destructive than TB v ISKP or TB vs national resistance whatsoever.
I based this opinion, based on researcher Tore Hamming research on the dynamics of another infamous jihadi groups, ISIS.
ISIS just like the Taliban, appears to be a coherent and solid group if viewed from the outside, but for people who did research enough like Tore Hamming, there are significant cracks within its ranks. One is the hardzore Hazimiya and the more moderate Binalists faction.
in one such cases in 2017 it results in a less known coups and counter coups in ISIS governorate across Iraq and Syria.
Taliban supporters may portray Taliban as solid groups, but the situation for the Taliban is even more perilous consider the Taliban had not two factions competing for power like ISIS did, it has multiple:
The Taliban move to be more inclusive now included in its ranks:
1.Islamists
2.Nationalists
3.Hanafists
4.Salafists
5.Shia
6.People that just want the US out
7.Al-Qaeda with international jihad agenda
8.groups which limit jihad agenda inside Afghanistan
9. Pashtun
10. Non-Pashtun
not to mention the many international player that will eventually fight, not cooperate for influence in Afghanistan such as:
1. Pakistan
2. Iran
3. China
4. Russia
and many more.
the recent reported killings of Salafists by the Hanafists and closure of Salafists seminary will eventually be the catalyst of conflict buried only by the common enmity against NATO occupation in the past. One such example in the past is the Indonesian independence war which saw former Indonesian independence fighter (communists, islamist and nationalists) turning their guns against each other right after the Dutch left in 1949.
So yeah we'll see where this goes.
Basically the Taliban now are NATO while ISIS is now playing the Taliban.