[IsTom]

Any useful TM program halts, so you need finite tape. Only figuring out beforehand how much will you need is difficult.

[brudgers]

You don't want your telephone exchange to halt. That's why the engineering marvel Erlang. "Engineering" being the key word. The Halting problem isn't a real problem. Just pull the power cord.

[IsTom]

You want every call handled in finite time. It's not that there is an unending computation in a telephone exchange, but a series of finite tasks. You can call it corecursion, coinduction or something else co-.