MS Teams: configure a phone number; Which licenses need to be applied to a user before enabling a user to use Direct Routing? a custom dial plan ?

 Use the Set-CsPhoneNumberAssignment cmdlet to assign a telephone number to the user.

A Microsoft 365 E5 or Office 365 E5 SKU including Teams Phone licenses, or individual add-on licenses provide can be used to apply the above licenses. Calling Plan functionality isn't required to use Direct Routing.

A dial plan is a named set of normalization rules that translate dialed phone numbers by an individual user into an alternate format (typically E.164) for purposes of call authorization and voice routing. They describe how phone numbers expressed in various formats are translated to an alternate format.