There are two components of PPTP:
TCP control connection between the two endpoints
an IP tunnel operating between the same endpoints. These are used to transport GRE encapsulated PPP packets for user sessions between the endpoints. PPTP uses an extended version of GRE to carry user PPP packets.
The control connection is established from the PPTP clients (for example, home users behind the NAT) to the PPTP server which is located on the outside of the NAT. Each session that carries data between the two endpoints can be referred as call. Multiple sessions (or calls) can carry data in a multiplexed fashion over a tunnel. The tunnel protocol is defined by a modified version of GRE. Call ID in the GRE header is used to multiplex sessions over the tunnel. The Call-ID is negotiated during the session/call establishment phase.