That sounds a lot like an authentication problem. Generally authentication problems are not confined to a particular area though.
You should have a look and see if your modem has useful logs. If the session actually starts and you get an IP address, then it's not an authentication problem, but a line quality problem.
If you turn the modem off for a few minutes, it will cool down, and the exchange port on the other end of the line will also reset. If the line quality problem is not serious, that will resolve it.
If your line is managed by Afrihost, there's a troubleshooter in the Afrihost client zone which walks you through all the possible self-help solutions before reporting a line fault.