After considerable thought I have decided to use land telephone lines either via direct connection or using an acoustic coupler. If you go the coupler route, and find that you too have to dial manually, you will probably also find that your modem refuses to co-operate at first, and maybe a message on the screen saying No Dial Tone, or similar. The answer is to add either X1 or X3 to your modem initialisation string. Both have the effect of telling your modem not to expect a dial tone. It makes no difference - as long as one or other of them in is the middle of the string somewhere, you should find that the modem is no longer expecting to hear a dial tone and goes ahead and emits its tones or pulses anyway.
Timing seems to be important.
And if your modem finishes and starts waiting for the other modem long before your manual dialling has connected, that too can often screw things up. Recently in Turkey, I could not get results through Istanbul. So I usually went through Greece, and learned to wait 15 seconds after I'd finished dialling Athens manually before I hit Enter to get my own modem started. But if Greece was bad for some reason and I changed to London, I had to hit Enter about half way through dialling London because the London connection went through almost immediately. Will soon be up the river to Sevilla for the winter.