It may be possible to share, but difficult. A serial connection to an ISDN device is inefficient. It too would be difficult to connect the rest of the phones in the house. The last option was an ISDN router. It would be central to the house and easy to wire.
Routers are very efficient.
It is an ethernet LAN device, so it is easy to share. However, a router is the most expensive alternative. Jeremy selected the Zyxel Prestige router. It allows the router to dial into Erol's using a standard PPP account, the least expensive Erol's account. There are several Prestiges, Jeremy selected the 128U, see Prestige 128 - Data Sheet .