My parents live rurally, and currently get their internet via Spark 4G cellular, across a huawei cell router - model B315s-608.  This router has telephone outlet that plugs into the existing copper in the walls, running three traditional handsets.  

The 4G is terrible.  Sometimes it is tolerable, but frequently not (as I type this, we're getting a solid 2.46 mbps download and 0.32 mbps upload).

I'd like to convert them to Starlink, but the constraint is that they want to keep their "Landline" and their number.  I found some providers who can absolutely allow them to use VOIP via starlink, and keep their existing number, but the one I contacted was requiring new (expensive) handsets - 3-off. 

Can anyone suggest a means of using starlink with the existing setup?  Could I use the existing modem-router as a router, with the existing telephone out line?  I guess I'd still need a "VOIP Provider"?